# insects

insects

Honey bee ( Apis mellifera )

Systematics
 without rank: Primordial mouths (protostomia) Over trunk : Molting animals (Ecdysozoa) Trunk : Arthropod (arthropoda) Sub-stem : Six-footed (Hexapoda) Class : insects
Scientific name
Insecta
Linnaeus , 1758
Subclasses

Insects ( Latin insecta ), also called insects or kerfe , are the species-richest class of arthropods (Arthropoda) and at the same time, with an absolute majority, the most species-rich class of animals at all. Almost a million insect species have so far been scientifically described (925,000 according to Grimaldi / Engel 2005, 865,000 according to Nielsen / Mound 1997). This means that more than 60 percent of all animal species described are insects. According to various extrapolations, however, one reckons with a multiple, with millions of undiscovered species being suspected, especially in the tropical rainforests . Fossil insects were found for the first time around 400 million years ago in the Devonian .

The word "insect" (from Latin īnsectum "incised") was Germanized in the 18th century and therefore means "incised (animal)", which refers to the strongly separated body parts. It is a loan translation of ancient Greek entomon (to "insect" ancient Greek ἐντέμνειν entémnein , German , cut ' ), which in Entomology ( entomology ) is included. The term "kerbtier" goes back to the German writer Philipp von Zesen . In the past, the scientific name Hexapoda (Greek for "six-footed") was used, which is now reserved for a higher-level group (see section on systematics).

## External anatomy

Scheme of the anatomy of insects
B - thorax (chest)
C - abdomen (abdomen)

1. Antenna
2. Ocellus (front)
3. Ocellus (top)
4. Complex eye ( compound eye )
5. Brain ( upper pharyngeal ganglion )
6. Prothorax
7. Back (dorsal) artery
8. Trachea
9. Mesothorax
10. Metathorax
11. First pair of wings
12. Second pair of wings
13. Midgut
14. Heart
15. Ovary
16. Hindgut (rectum)
17. Anus
18. Vagina
19th century . belly sided nervous system ganglia
20th Malpighian tubules
21 Tarsomer
22 pretarsus
23 Tarsus
24 tibia
25 femur
26. trochanter
27 foregut
28 Thoraxganglion
29 coxa
30. salivary gland
31 subesophageal
32. mouthparts

The size of the insects varies greatly and is between one and 20 millimeters for most species. The smallest species are hymenoptera ( dwarf wasps ) parasitizing in the eggs of other insects , the male of the smallest known species is 0.15 (to 0.24) mm long. The smallest free-living insects are dwarf beetles with a body length of 0.4 mm (even smaller figures are based on measurement errors). The largest known insects are stick insects with a body length of about 33 centimeters and the giant longhorn beetle Titanus giganteus with a body length of 15 centimeters. Depending on their way of life, the body of the insect can be elongated, flattened or more or less spherical.

All insects have in common an exoskeleton made of the solidifying structural protein sclerotin and the polysaccharide chitin as well as the anatomically mostly clearly visible division into the three sections ( Tagmata ) head (caput), chest ( thorax ) and abdomen ( abdomen ) and the presence of exactly three pairs of legs . All body parts consist of individual segments that have a back plate ( tergite ), a belly plate ( sternite ) and side plates ( pleurite ). To the side of the individual segments, openings of the tracheal system , so-called spiracles , are created in the basic construction plan, the actual number of which, however, can vary greatly with the individual taxa of the insects. Extremities and their parts ( mouthparts , styli , gonopods ) can also appear in pairs on each individual segment. The thorax has six legs (three pairs). Each segment also has a pair of ganglia in the abdominal cord , with the ganglia of the head fused to form an upper pharyngeal and a lower pharyngeal ganglion. Between the individual segments there are elastic membranes, the intersegmental membranes , which enable the segments to move relative to one another and to change the volume of the body during egg production, eating or breathing. In the case of rigidly fused segments, for example in the head, these membranes are not present.

The exoskeleton forms the outer layer of the insect's body. This cuticle is formed by an underlying epidermis . The sensory organs and various glandular exits of the insect are embedded in it. With greater or lesser involvement of the epidermis, the exoskeleton forms various surface structures, including warts, thorns, hair, bristles, scales and bumps. The outer layer of the insect or individual parts of the body can be colored due to the storage of dyes ( pigment colors ) or due to special light-refracting surfaces ( interference colors ).

The main sensory organs used are hairsensils , which are distributed over the body. These react to shocks and vibrations, but can also perceive odors, moisture or temperatures. Some of these sensory cells are grouped into sensory organs, such as the Johnston organ on the pedicellus of the antenna or the tympanic organs for sound perception, which are found, for example, in long-feeler terrors . This makes it possible to perceive vibrations in the range from 1 Hz to 100,000 Hz. The compound eyes and the ocelles serve as optical sensory organs, and in many larvae also the larval point eyes.

### The insect head: eyes, antennae, mouthparts

Head of a dragonfly with compound eye and ocelles

The insect head consists of a front section, the segmental nature of which is controversial (according to the previously prevailing Articulata hypothesis, it corresponds to the acron of the annelids) and five other fused segments and carries the eyes and the limb antenna as well as a typical apparatus of mouthparts made of paired mandibles and Maxillae and an unpaired labium . Accordingly, the mouth opening through which food is taken is also located on the head. The seams of the insect's head visible from the outside have nothing to do with the segment boundaries with one exception, the boundary between the fifth and sixth segment ( postoccipital suture ).

The compound eyes are located on the front section, next to them insects have three point eyes ( ocelles ) in the basic construction plan for light and dark perception. The second segment is the antenna segment with the flagellated antenna typical of the insects . This differs from the articulated antenna that is found in the original hexapods in that it has only one basal or shaft member with muscles, the scapus . The following turning link, the pedicellus , is movable together with the flagellum compared to the scapus. In the pedicellus there is Johnston's organ, a sensory organ that can react to vibrations and sound. The scourge itself has different lengths and can also be shaped very differently. It has sensory cells that primarily serve to perceive smells.

Harvest ant in the scanning electron microscope

The third segment is the intercalar segment , which again has no structures of its own (it corresponds to the second antenna segment of the crustaceans ). On the fourth to sixth segment are the mouthparts, beginning with the mandibles and maxillae, which are each paired, and finally with the labium, which is unpaired due to an adhesion of the basal phalanx and closes off the oral cavity to the rear. The front end is formed by the clypeus with the labrum . The mandible consists of only one member. As a biting and chewing tool, it is the strongest mouth tool in most insects. The maxilla shows the extremity character of the mouth parts very clearly and is reminiscent of the split bones of crustaceans. It consists of a basal segment, the coxopodite , which is divided by a cardo and a stipes . This is followed by two Kauladen , as Galea and lacinia be called, as well as a multi-element probe , the palp maxillary . The labium corresponds to this structure, with the exception that the two coxopodites are fused here and form a common basal plate, which is divided into mentum and submentum . Here, too, there are two chews on each side, the glossae and the paraglossae , as well as a palpus, the palpus labialis . The palps in particular have sensory cells for smell and taste perception. Further structures of the oral cavity are the hypopharynx as a tongue-shaped formation of the oral vestibule and the epipharynx on the inside of the labrum.

The construction plan described here corresponds to the basic pattern of the insects and is called the chewing-biting type. By modifying the individual structures to adapt to different diets, this type can develop into various other types of mouthparts, including the licking-sucking mouthparts of many flies or the stinging-sucking mouthparts of mosquitoes or bedbugs .

### The thorax

Diced dance fly ( Empis tesselata )
Hairiness of a bumblebee

The thorax always consists of three segments and carries the six walking legs in all insects and the two pairs of wings in winged insects. The individual segments are referred to as prothorax , mesothorax and metathorax due to their position relative to one another . Each of these segments carries a pair of thoracic legs, commonly shaped as walking legs . Depending on their function, these legs can be designed very differently, but always have the same basic structure. They consist of a hip ( coxa ), a subsequent thigh ring ( trochanter ), the thigh ( femur ), the splint ( tibia ) and an articulated foot ( tarsus ), which consists of one to five tarsal members and a praetarsus. The praetarsus normally has two claws and, in the case of many insects, several other structures that are primarily used to hold onto various surfaces.

In the winged insects, the meso- and metathorax each have a pair of wings , which can be differently developed in the different groups of insects. Since the meso- and metathorax form a unit here, they are jointly referred to as pterothorax (from the Greek pteron = "wing, feather, fin").

The original insect groups such as the rock jumpers and the little fish are, however, wingless, accordingly wings are not included in the basic plan of the insects and only emerged later within the insects, with the development of the flying insects . Instead, in the basic plan, the insects have duplicates of the epidermis on the sides of the thorax segments, which are known as paranota and are regarded as the forerunner structures of the wings.

### The abdomen

The abdomen originally consists of eleven segments, of which, however, in the various insect taxa, individual segments have fused or have been lost. The abdominal segments only carry modified extremities and no real legs. For example, styli can be found , especially in larval forms. In addition, a number of insect groups have cerci on the last segment, which are designed for different functions. The gonopods, which form the mating organs of the males and the ovipositor of the females, are also transformed extremities. In contrast to the thorax, the abdomen contains very few muscles; instead, most of the organ systems are located here.

## Internal anatomy and physiology

### Nervous and endocrine systems

Insect nervous system
( A termite , B swimming beetle , C fly )
1  upper  pharyngeal ganglion
2  sub pharyngeal ganglion
3 other nerve nodes

The insect's nervous system essentially corresponds to the common basic structure of the mandibulata , which includes crustaceans and millipedes as well as them . It consists of a pair of abdominal cord that has a pair of ganglia in each segment. In the head, the ganglia are fused to form an upper pharyngeal ganglion, the brain, and a sub- pharyngeal ganglion, both of which are connected to one another via the pharyngeal connector . The brain itself has a very large front area, the protocerebrum , from which areas on both sides radiate into the centers of the complex eyes, which are called lobi optici . The ocelles are also innervated by this part of the brain. From the deuterocerebrum , the second section of the brain, nerves pull into the antennae and the tritocerebrum supplies the foregut with nerves through a tritocerebral commissure. The mouthparts are supplied by the fourth to sixth segment ganglia, which are concentrated in the sub-canal ganglion. The abdominal marrow runs through the thorax in the form of a typical rope ladder nervous system , with three particularly large pairs of ganglia in the thorax supplying the legs and (if present) the wings. In the basic plan, the abdomen contains 7 normal pairs of ganglia and a ganglion mass in the eighth abdominal segment, which consists of the ganglia of all the following segments.

The autonomic nervous system consists of three sections. The front section is formed by the nerves of the mouth and foregut area with the frontal , hypocerebral and ventricular ganglion as well as the corpora cardiaca and the corpora allata . The second section is formed by the paired ventral ganglia chain, which is responsible for the innervation of the spiracles. The rear section of the intestine and the genitals are supplied by the caudal nerve. In the brain in particular, insects also have neurosecretory glands, which also include the already mentioned corpora cardiaca and corpora allata. The latter release the juvenile hormone , which influences the developmental stage during the moult . The moulting itself is induced by the moulting hormones, especially the ecdysone .

### breathing

The insects' respiratory organs are largely rigid tubes that run through the entire body and extend in ever finer tubes to the organs and individual cells. This system is known as the tracheal system and develops inward from the invaginations of the epidermis . Correspondingly, like the outside of the insect, the trachea are covered with an epidermis and a lining of chitin. This cuticle is stiffened by taenidia , spiral structures that wrap around the tube to prevent collapse. The tracheae open outward in breathing openings known as spiracles.

The breathing process is primarily passive. This type of breathing also means that insects can get larger in warmer areas. In rather cold areas, the molecular movement of the air is insufficient to supply the inner trachea with sufficient oxygen. However, air exchange is regulated by muscular control of the spiracles and can be increased by ventilation. In many insects the number of stigmas of originally a pair on each thoracic and abdominal segment has been reduced by cross connections ( anastomoses ), and in many taxa air sacs have been developed to support breathing and to store oxygen. In aquatic insects, further adaptations have also developed that allow breathing underwater. In many water beetles and other water insects, for example, there is an area that has a special surface to hold an air bubble ( physical gill ). Other types have a constant-volume plastron or breathing tubes for breathing. In the case of aquatic insect larvae in particular, tracheal breathing is completely dispensed with and oxygen is instead absorbed through the tracheal gills or the skin.

### blood circulation

The blood vessel system of insects essentially consists of a cavity, the haemocoel , in which all internal organs swim in blood ( hemolymph ) (open blood circulation).

As the remaining part of the blood vessel system, insects have an unbranched, simple dorsal vessel. This has a contractile part in the abdomen that pumps the hemolymph into the body and is accordingly called the heart ( closed caudally ), and a branch that supplies blood to the head, the brain and the head and aorta ( open cranially) ) is called. The blood flow is kept in motion by the peristalsis of the heart, while the hemolymph is sucked into the heart from one to twelve pairs of lateral openings in the heart, the ostia , and pumped forward through the aorta into the head. From there it spreads through the thorax into the abdomen and also into the legs and wings of the animal. Smaller pump systems on the legs, antennas and wings ensure that the supply reaches the ends of the structures. There is a muscular so-called antenna heart in the head, it pumps the hemolymph into the antenna ampoules and the corresponding antennae. Below the heart lies a dorsal membrane made of muscles and connective tissue, which acts as a diaphragm separating the back of the body ( pericardial sinus ) from the main body ( perivisceral sinus ) with the intestines and the genital organs. Another diaphragm lies above the abdominal part ( perineural sinus ) with the abdominal marrow.

The hemolymph itself makes up 20 to 40 percent of body weight and consists of cells, hemocytes , and liquid plasma. It transports the carbon dioxide , proteins , nutrients, hormones and excreta produced during breathing and also serves to regulate the osmosis and maintain internal body pressure. The blood is usually colorless, but it can also be greenish-yellow; not only does it transport nutrients to all parts of the body, but it also brings the hormones to their destinations. The blood contains special cells that fight pathogens and others that repair damage or wounds. Unlike vertebrate blood, insect blood does not contain cells involved in breathing.

The traditional view that the blood of insects has lost its function of oxygen distribution due to the effective tracheal system has been put into perspective in recent years due to new findings. The red blood pigment hemoglobin has long been found in aquatic insect larvae that live under oxygen-deficient conditions, such as B. mosquito larvae are known. Genome analyzes have now shown that hemoglobin production is one of the basic assets of almost all insects. Hemoglobin occurs primarily in the gills and other respiratory organs, so that its function for breathing is ensured. The blue blood pigment haemocyanin was also detected in numerous insect orders ; at least for a stonefly, a function for the oxygen supply was also demonstrated here.

### Digestion and excretion

Catchy from Australia: Ditaxis biseriata

Digestion takes place in the animals' intestinal system. This is divided into three functional sections, which are called fore , middle and rectum according to their location . The anterior and posterior parts are formed ectodermally and have a lining with a chitin cuticle, which accordingly also has to be skinned. The foregut begins with the opening of the mouth and consists primarily of the muscular pharynx . This section is connected to the midgut via the esophagus , which in many insects can also contain areas for food storage (ingluvies) or a forestomach (proventriculus) with chewing structures such as ridges made of chitin. The midgut is lined with a glandular epithelium and produces the enzymes that are necessary for digestion. This is also where the absorption of nutrients takes place , which in many insects takes place in blind tubes (caeca) or crypts. In the caeca and crypts, endosymbiotic microorganisms ( bacteria , fungi or flagellates ) can also live in many insects , which are required for the breakdown of certain food components, e.g. B. for the breakdown of cellulose . In addition, an endogenous cellulase was detected in a few insects (in the termite species Reticulitermes speratus and Coptotermes formosanus ). The origin of the animal cellulase gene is seen in the last common ancestor of the Bilateria .

The indigestible remains ( excrement ) are excreted through the rectum. The excretion of the insects takes place via small blind tubes which open into the intestine at the transition from the midgut to the rectum. These are called Malpighian vessels and, like the rectum, are of ectodermal origin. In the cells of these tubes, nitrogen-containing excreta are actively extracted from the hemolymph and excreted with the excrement. In the rectal papillae, water is withdrawn from the excretion products before excretion.

The fatty bodies , which are large lobes in the insect's abdomen, are important structures for storing nutrients and excrement . In addition to storage, they are used for the synthesis of fats and glycogen as well as the breakdown of amino acids .

### Genital organs

With the exception of a few species, all insects are separate sexes. Very few species are hermaphrodites ; a number of species reproduce through parthenogenesis .

Mating of the blue-winged demoiselle

The males have paired testicles for sperm production in the abdomen, which are connected to paired seminal vesicles (Vasa seminales) via spermatic ducts (Vasa deferentia). These lead to an unpaired or paired ejaculation duct , the ductus ejaculatory , and then to the outside via more or less complex reproductive organs, the aedeagus , mostly in the ninth abdominal segment. In addition, there may be additional glands that form seminal fluids or substances for the formation of spermatophores and add them to the sperm.

The ovaries of the females are also usually created in pairs. They mostly consist of a clump of individual ovarian cords, which are known as ovarioles . Each of these consists of a ovarioles germarium in which the eggs are produced, and a Vittelarium to produce the yolk cells . Depending on the way in which the eggs are supplied with yolk, a distinction is made between three different forms of ovarioles, which can occur in different insect taxa. In the first form, called panoistic ovariole, individual eggs in the Vitellarium are supplied with yolk. In the meroistic-polytrophic type, each individual egg cell has several nutrient cells and receives the yolk through them. In the case of the meroistic-telotrophic ovariole, the individual egg cell remains connected to the germarium via a nutrient cord and receives the yolk in this way. In all types of egg cells, follicle cells are placed around the growing egg cells .

The ovarioles unite and end in an unpaired vagina , which ends between the seventh and ninth abdominal segments either directly outside or in a mating pouch, the bursa copulatrix (exception: mayflies). In the area of ​​the vagina, almost all insects have a sperm collection pocket, the receptaculum seminis , and various attachment glands for the production of cement substances or the like can also be present.

## Habitats

Hissing cockroach , endemic to Madagascar

With the exception of the oceans, insects can be found in almost all habitats and areas on earth. The greatest biodiversity exists in the tropical areas , while in extreme habitats such as the polar regions , the high mountains and the coastal marine areas there are only very few, but highly adapted, insect species. Thus one finds approximately in the Antarctic , the chironomid Belgica antarctica , on the sea surface to the bugs counting halobates and tidal watts, the larvae of midges of the genus Clunio .

Some species are very specialized and therefore only occur in particularly suitable habitats ( stenöke species), others, on the other hand, can live in almost all habitats with the exception of extreme habitats ( euryöke species) and were partly distributed worldwide by humans, so that they are now cosmopolitans are. The latter mainly include various types of cockroaches , ants and termites as well as honey bees kept as livestock .

Most insects live in the ground or on structures close to the ground and on and in plants. It is assumed that around 600 species of insects are associated with each species of tree in the tropical rainforests, with an insect number of around 30 million for 50,000 tree species. A number of insects also live on animal species, mostly as ectoparasites such as the various types of animal lice and fleas or as commensals and hunters in the fur of the animals. Humans are no exception here, as the various types of human lice can be found on them . Insect endoparasites in animals are less common . Mention may be made especially to the here Diptera belonging warble flies in which the larvae in the pharynx ( cephenemyiini ), the nasal cavity ( Nasendasseln ) or even in the stomach ( gasterophilinae ) develop herbivores.

## Way of life

Lacewing larva prey on an aphid

Because of their diversity, insects have now found almost any ecological niche appropriate to their size . A large number of species play an important role in the remineralization of organic substances in the soil, in the litter, in dead wood and in other organic structures. This group also includes the body decomposers found in animal corpses. Many other species live as herbivores on living parts of plants, the spectrum ranges from root hairs and wood to leaves and flowers. A number of species live as nectar and pollen collectors and play an important role in plant pollination . Still other insects live in and on mushrooms and feed on them. A large group of insects predatory feeds on other insects or smaller prey. The last group are those insects that feed on parts of larger animals such as hair, scales and the like. This group also includes the numerous parasites among insects that, for example, suck blood or develop in living tissues.

A specialty within insects are different types of state-forming insects . This form of coexistence has developed several times independently of one another in termites and various hymenoptera ( ants , bees , wasps ). With these animals, an insect state is built up, in which the individual animals assume certain roles within society. This often leads to the formation of castes , the members of which are morphologically and in their behavior similar. For example, many ants have workers, soldiers and nest keepers. In these cases, only very few sex animals take on reproduction within the insect state, sometimes only a single queen who lays fertilized and unfertilized eggs.

## Reproduction and development

In the case of insects, sperm transmission originally takes place via spermatophores, i.e. sperm packets. In the case of the rock jumpers and the fish, these are placed on the ground by the males and taken up by the females. In all of the following groups of insects, there is direct sperm transfer through copulation , in which the sperm are introduced directly into the vagina or the bursa copulatrix and either fertilize the egg cells or are forwarded to the receptaculum seminis for storage. Most insects lay eggs after mating ( ovoparia ), others are ovoviviparous , so they incubate the eggs while they are still in the body until they are ready to hatch . It is even more rare that fully developed larvae ( larviparia ) or even pupae ( pupiparia ) are born.

The furrowing is superficial in most insects . This means that the very yolk-rich egg with a central yolk (centrolecithal egg) forms a furrow center from which the furrow starts. In its area, several daughter nuclei form with surrounding plasma (furrow energies), which divide into a single-layer blastoderm as a covering epithelium or serosa around the yolk. A germinal system then forms in the ventral area, which grows into the yolk as a germ line and forms a cavity (amniotic cavity). The main germ formation takes place in this cave, after which the germ unrolls outwards again and the back of the animal can be closed over the yolk. The eggs then hatch larvae (juvenile stages with their own larval characteristics) or nymphs (juvenile stages without their own characteristics).

Hornet emerging from its pupal shell

Outside the egg, the postembryonic development follows, according to which insects are classically divided into hemimetabolic and holometabolic insects . However, only the latter is also a taxonomic group, since the hemimetabolic development corresponds to the original state. The development is very different in the different groups and depends very much on the lifestyle of the juvenile stages and the adults . In all hemimetabolic insects, the adult animal, the imago, develops through a different number of larval or nymph stages without a pupal stage. There is always a molt between the individual stages, during which the old cuticle is thrown off and a new one is created. Depending on the concentration of the juvenile hormone in the blood, there is either a molt from one larval form to the next (if there is a lot of juvenile hormone) or from a larval form to an imago (if there is little juvenile hormone). Another hormone, ecdysone, determines the time of the moult . When moulting, the animals primarily grow, and individual features are newly created. This takes place through histolysis of individual structures and the formation of imaginal systems or the eversion of special imaginal discs .

Fire bugs , larvae and adults

In most hemimetabolic insects such as the various forms of locusts or bedbugs , the nymph is basically similar to the adult animal and, apart from the missing wings, has no special larval adaptations. In contrast, there are also hemimetabolic insects with real larvae, for example dragonflies or mayflies . A finer subdivision of the hemimetabolism is possible. For example, one speaks of a palaeometabolism (small fish, rock jumpers) when the larvae have hardly any characteristics of their own and only develop into an imago by changing their size. The heterometabolism is mainly characterized by a step-by-step development of the wings and occurs in dragonflies, stone flies and most Schnabelkerfen. Finally, there is neometabolism , in which the wing systems are only created in the last two larval stages; this is the case with some cicadas and the fringed winged birds .

Holometabolic insects go through a metamorphosis , starting from the egg via the larva to the pupa and then to the adult animal ( imago ). The larva often does not have the slightest physical resemblance to the imago and has a number of characteristics of its own, including different habitats and food sources, compared to the imago - a not inconsiderable ecological advantage of the holometabola.

## Population dynamics

The population dynamics of many insect species has an approximately discrete character: In the temporal development of a population, mutually delimited, non-overlapping generations can be identified.

If one writes for the size, or after normalization abundance , of a population at the point in time and for the number of offspring generated per individual, then for the simplest possible description of a population dynamics results: ${\ displaystyle N_ {t}}$${\ displaystyle t}$${\ displaystyle r}$

${\ displaystyle N_ {t + 1} = rN_ {t}}$

If the proportion of the offspring that is self-reproducing at the time depends on the size of the population at the time , one obtains a functional relationship of the form: ${\ displaystyle t + 1}$${\ displaystyle t}$

${\ displaystyle N_ {t + 1} = f (N_ {t})}$

with a function . Various concrete approaches are possible for this , using the Hassel equation , for example . A direct dependence of fertility on population size leads to a similar formalism . ${\ displaystyle f (\ cdot)}$${\ displaystyle f (\ cdot)}$

## Fossil record

Fossil jewel beetle from the Messel Pit

The earliest widespread an insect associated fossil is Rhyniognatha hirsti from the Pragian , a stage of the Devonian approximately 407 million years ago. The oldest springtail, Rhyniella precursor , comes from the same formation and was considered the oldest insect as long as it was counted among the insects. These relatively advanced fossils suggest a long evolutionary history that probably goes back to the Silurian. Insects are thus probably only slightly younger than the oldest finds of terrestrial animals. Since the relics of Rhyniognatha that are difficult to interpret, including presumably mandibles, have survived, statements about what it looked like and what kind of relationship it would be today are speculative. The first writers even consider a connection with winged forms to be conceivable, which, however, remains very speculative. Recent studies even suggest that it was actually a myriapod. Actual fossil finds of winged insects are about rocks of the lower Carboniferous-Upper Carboniferous border , that is, they are about 95 million years younger. (Older Devonian finds, described as Eopterum devonicum , were later found to be the remains of the carapace of a crab.) Winged insects are present here from a variety of orders, including both living and extinct. The wing morphology already allowed different flight styles. The Carboniferous fauna included z. B. the Palaeodictyoptera , which sometimes had small wing-like appendages on the first thoracic segment. During the Carboniferous these ancient orders made up about a third of the insect fauna; Cockroaches were represented in a large number of individuals, but not particularly rich in species. Of the 27 insect orders recorded in the Carboniferous, 8 became extinct at the transition to the Permian or a little later. Three other orders died out at the transition to the Triassic. Since this evolutionary bottleneck , all surviving lines have increased more or less steadily in diversity up to the present day, and the third mass extinction on the Cretaceous / Tertiary border also seems to have only slightly affected the insects. Surprisingly, it is not faster speciation rates, but rather reduced extinction rates, that seem to be more important for this increased diversity.

Both radiations as well as longer periods with low species numbers and little evolutionary innovations can be associated with pronounced fluctuations in the oxygen level of the earth in the Paleozoic Era, extended periods with very few fossil finds are therefore probably not exclusively due to coincidences in fossil records, but can be real be. During the periods of unusually high oxygen levels, there lived spectacular giant insects with sizes that are up to ten times larger than all recent forms. The primeval dragonfly Meganeura monyi (found in Commentry, France) achieved a wingspan of 63 cm, Meganeuropsis permiana is said to have had a wingspan of 71 cm, probably close to the upper limit of the arthropod blueprint for flight ability.

In Perm , most modern insect orders appeared, but just the dominant today played holometabolous insects long time a subordinate role, and only from the Triassic occurred in this a true explosion of the species and variety of shapes. Reasons for this success are seen in the mass extinction at the end of the Permian (previously dominant insects died out), in better resistance to the arid climate of the Permian and in the spread of seed plants .

All currently recognized insect orders and almost two thirds of the families are also fossilized, whereby their tertiary species numbers correlate well with their current ones (exception: butterflies, which are fossilized less species-rich).

## Systematics

Three groups that were traditionally counted among the insects as urine insects, the springtails (Collembola), double tails (Diplura) and leg taster (Protura) are no longer considered actual insects. They are listed here together with these within the parent group of the hexapods (Hexapoda). Both the Hexapoda and the insects as such are regarded as secure taxa due to their typical characteristics ( apomorphies ) . The actual sister group of insects within the Hexapoda is, however, controversial.

### Classic system

The insects are often compared as free kitties (Ectognatha) to the sack pus (Entognatha), which consist of the named taxa of urine insects. The most important characteristics of their own are a flagellated antenna with a base member ( scapus ) and a pedicellus with a Johnston organ . The actual scourge went back to the original third link of the antenna, which was divided into a different number of scourge segments. Further apomorphies are the possession of the back branches of the tentorium , a skeletal structure in the head, paired claws on the praetarsus with an articulated connection to the same. Even the primary structured in five Tarsenglieder Tarsus and the transformation of the rear Abdominalextremitäten to gonopods and participation at the same ovipositor ( ovipositor ) of the females is considered a newly acquired trait. The original eponymous feature, the exposed jaw, is, however, an original feature ( plesiomorphism ), which was already present in the common ancestors of the crustaceans , centipedes , millipedes and hexapods ( mandibulata ).

Within the insects, in the classical system, different numbers of subclasses, superordinates and orders are differentiated depending on the source. This is due to the fact that the insect groups are actually related to one another, which is still not fully understood, and to the different views of the descriptors about the arrangement within a classic hierarchical description. The following system is to be followed in this work; Differences to other works arise above all in the rank of dragonflies and mayflies, which are often classified as an order, as well as in the Schnabelkerfen, whose subordinate orders are listed in some works as separate orders, as well as in the grasshoppers , which are often still the short-feeler terrors and the long-feeler terrors are classified as orders (instead of as subordinate orders as here).

### Phylogenetic systematics

The phylogenetic system tries to build the system on the basis of the actual relationship. The main differences to the classic system are the treatment of the so-called paraphyletic groups. Paraphyletic taxa are only delimited by ancient features, symplesiomorphies . They do not include all descendants of an actual common ancestor. The most important method is still a careful study of the anatomy and morphology, including significant fossils.

As an additional method, molecular trees are of great importance today . Molecular pedigrees are based on the direct comparison of homologous DNA sequences . For this purpose, the same gene segments of the different organisms for comparison are copied, sequenced and then compared with the aid of the polymerase chain reaction . Using various statistical techniques, the sequences are arranged in a molecular family tree, which ideally reflects exactly the graded relationship of the individuals examined. Frequently used genes for the molecular family tree of insects are genes for ribosomal RNA , histone H3 , elongation factor 1 alpha 1 and various mitochondrial genes (e.g. mt-CO1 ), but increasingly also the entire mitochondrial DNA . A comparison on the basis of the entire sequenced genome would of course be the most meaningful; it has actually already been undertaken due to the few completely sequenced insect genomes. Like any other method, creating molecular trees has its difficulties and pitfalls. As is often the case with new methods, the initial euphoria has given way to a certain disillusionment because too many family trees drawn from different genes or using different methods yielded starkly contradicting results. A main problem is that many insect orders evidently emerged in rapid succession in a rapid radiation and then diverged over more than three hundred million years, so that the information from the original radiation can hardly be broken down.

Current proposals for phylogenetic systematics are usually based on a combination of morphological and molecular pedigrees. In addition, so-called supertrees are set up that bring together the results of numerous studies without contributing new data themselves. The family trees, which are currently intensely discussed (including :), do not agree on numerous points. Nevertheless, there have been numerous improvements compared to previous systematics (classical basis :) created on a purely morphological basis.

The phylogenetic system manages without the ranks of the classical systematics above the species. As a rule, the insect orders are retained as kinship groups, even if it is clear that they can neither have the same age nor the same actual rank. The following classical orders have undoubtedly proven to be paraphyletic:

• Cockroaches, relating to termites (most likely sister group: genus Cryptocercus )
• Dust lice, in relation to animal lice (most likely sister group: book lice (Liposcelidae)).
• Beaked flies, in relation to fleas (most likely sister group: Winterly (Boreidae)).

The second group in each case is therefore not actually a sister group of the former, but a part of it (i.e. it is more closely related to some of its families than they are to each other). The monophyly of the other orders was confirmed, sometimes after doubts arose in the meantime (especially beaked flies in relation to two-winged and large-winged flies in relation to camel neck flies: not yet certain).

The system presented below (combined according to the above sources) is uncertain, controversial in various places and may change in the next few years. It roughly reflects the current state of knowledge. To further resolve the relationships within the Polyneoptera (whose monophyly is almost undisputed today) is currently impossible with sufficient certainty. Some traditionally very controversial questions, such as the position of the fan-winged or the ground lice, seem to be nearing a solution.

insects

Dicondylia

Flying insects

Metapterygota

New wingers
Polyneoptera

Dictyoptera

Notoptera

NN

Grasshoppers

Eumetabola
Paraneoptera
 Bird lice and animal lice

Holometabola

NN

Neuropterida

Neuropteriforma

Coleopteroid

Mecopteroida
Amphiesmenoptera

Antliophora

NN

## meaning

### As beneficial insects

The main livestock are honeybees , parasitic wasps for biological pest control , the caterpillars of the silk moth and, in many countries, locusts and crickets as food ( food insects ). The consumption of insects is widespread over large parts of Africa, Southeast Asia and Central and South America and is known as entomophagy . Insects are a protein-rich diet; around 500 species are used for human consumption worldwide. However, a large number of insects also play a major role in human food production as secondary plant pollinators, since without them fruiting could not take place.

Some insects are also kept in terrariums as pets . Particularly noteworthy are the ants that are kept in formicariums . They belong to the hymenoptera and are therefore close relatives of bees , wasps and hornets . Other popular insects among terrarium keepers are stick insects, catch terrors and various species of beetles and cockroaches, and mealworm larvae (mealworms), crickets and other insects are bred as food for reptiles and other pets and as fishing bait . A development in the last few decades has been the keeping of butterflies in large greenhouses , which are opened to visitors as butterfly houses and in some places become the sights of the region.

Various insects are also used in the pharmaceutical industry , the best-known example being the Spanish fly ( Lytta vesicatoria ) and some other oil beetles that produce the substance cantharidin . Drosophila melanogaster and various species of grasshoppers and beetles have established themselves as scientific experimental animals . The corpse decomposers among insects, especially the larvae of various flies and beetles, also play an important role in criminology today . The research branch of entomological forensics for the investigation of criminal cases is based on the research of these animals. Blowfly larvae are used in wound healing . In addition, various types of scale insects are used to produce dyes, varnishes or waxes, such as B. carmine (cochineal) and shellac .

The Ministry of Defense of the United States was looking in 2006 for ways to equip insects even during metamorphosis with microtechnology and as a military "insect cyborgs use".

The role of insects for the function of all terrestrial ecosystems and especially the metabolism of substances can normally hardly be estimated because of their ubiquity. It becomes clear, for example, in the disposal of discarded organic waste. Researchers from North Carolina State University in Raleigh (USA) presented, for example, in a study published in the fall 2014 survey found that insects on 24 landscaped road median strip in the New York district of Manhattan a year per center strip, about 4 to 6.5 kg food waste ' dispose of ". The most important “disposer” was an introduced neozoic ant species.

### As pests

Anopheles sucking blood

A number of insects as pest ( bugs ) to commercial and ornamental plants , wooden structures and products ( wood preservative known) and food supplies, others live as parasites on humans and their pets . Furthermore, some insect species, especially blood-sucking insects occurring in large numbers , are vectors ( vectors ) of diseases such as the rat flea for the plague , mosquitoes of the genus Anopheles for malaria or the tse-tse flies for sleeping sickness .

Infestation with plant pests can lead to major crop failures in today's monoculture of useful plants. In addition to the direct damage caused by eating, many insects also transmit plant diseases, mostly viroses and fungal diseases. Many other pests also live as stored pests in harvested or already processed materials and lead to further damage here. On the other hand, there are other types of insects that are used for biological pest control and that hunt pests directly or lay their eggs in them. Various parasitic wasps in particular are used in a targeted manner against specific pests.

## Insect death

Since the end of the 1980s there has been talk of insect death in Germany, which refers to both a decline in the number of insect species ( biodiversity ) and the number of insects in an area ( entomofauna ). In early 2019, Australian researchers published a global meta study . They evaluated 73 studies on insect mortality from different regions of the world and came to the conclusion that there had been a massive decrease in the biomass and biodiversity of insects in the previous decades.

## Materials on insects

Meat fly cleaning itself (4:05 minutes film) - 6 MB Xvid in Ogg -Container
The film shows a meat fly in Portugal, which cleans its wings and its head with the help of its front and rear legs. In order to be able to see the very fast movements of the fly better, the film is played back at half speed.

### Museums / collections

In order to record the biodiversity and the range of variation in insects, insect collections are created by private collectors and scientific institutions. Museum collections in particular achieve considerable sizes and numbers of individuals. The scientific insect collection in the Natural History Museum of the Admont Abbey ( Styria ) by Father Gabriel Strobl (1846–1925), for example, contains around 252,000 specimens from 57,000 different species, the Diptera collection, with around 80,000 preserved specimens and around 7,500 different species names, is one of the three most important bow ties collections in Europe.

Private collectors also often make collections of insects; Especially conspicuous butterflies and beetles are often collected because of their aesthetic preferences or rare species because of their rarity. A collection for purely aesthetic reasons is not advisable, as especially the populations of rare butterflies such as the European Apollo butterfly can be brought to the verge of extinction due to the passion for collecting (in addition to their habitat loss and other reasons). On the other hand, especially in tropical countries, there are economic interests in the passion for collecting, which is particularly popular in Europe and North America. There butterflies and other popular insects are grown in special farms and sent to collectors.

### Cultural meaning

Especially useful insect species such as honeybees and other species living in their immediate vicinity have found their way into human culture. The best-known processing of the insect motif in literature is Maya the Bee and Her Adventures by Waldemar Bonsels . The Mexican folk song La Cucaracha and The Flight of the Bumblebee are particularly popular in music . The latter also found its way into heavy metal through the band Manowar . In Germany the nursery rhyme Maikäfer flies ! long been a cultural common property.

## literature

Commons : Insects  - Collection of images, videos and audio files
Commons : Insects as a theme  - collection of images, videos and audio files
Wiktionary: Insect  - explanations of meanings, word origins, synonyms, translations

## Individual evidence

1. Duden "Etymologie" - u. a. with Latin insecta; Retrieved July 10, 2017
2. David Grimaldi, Michael S. Engel: Evolution of the Insects. (= Cambridge Evolution Series ). Cambridge University Press, 2005, ISBN 0-521-82149-5 .
3. ^ Ebbe Schmidt Nielsen , Laurence A. Mound: Global Diversity of Insects: The Problems of Estimating Numbers. In: Peter H. Raven, Tania Williams (Eds.): Nature and Human Society. The Quest for a Sustainable World. Proceedings of the 1997 Forum on Biodiversity. National Research Council USA, Board on Biology, 1997.
4. Edward L. Mockford: A new species of Dicopomorpha (Hymenoptera: Mymaridae) with diminutive, apterous males. In: Annals of the Entomological Society of America. Volume 90, 1997, pp. 115-120.
5. Vasily V. Grebennikov: How small you can go: Factors limiting body miniaturization in winged insects with a review of the pantropical genus Discheramocephalus and description of six new species of the smallest beetles (Pterygota: Coleoptera: Ptiliidae). In: European Journal of Entomology. Volume 105, 2008, pp. 313-328.
6. A. Kaestner: Textbook of Special Zoology. Volume 1: Invertebrates. 3rd part: A. VEB Gustav Fischer Verlag, Jena 1972, p. 21.
7. recent literature summarized in: Gerhard Scholtz, Gregory D. Edgecombe: The evolution of arthropod heads: reconciling morphological, developmental and palaeontological evidence. In: Development Genes and Evolution. Volume 216, Numbers 7-8, 2006, pp. 395-415. doi: 10.1007 / s00427-006-0085-4
8. Thorsten Burmester, Thomas Hankeln: The respiratory proteins of insects. In: Journal of Insect Physiology. Volume 53, Issue 4, 2007, pp. 285-294. doi: 10.1016 / j.jinsphys.2006.12.006
9. Christian Pick, Marco Schneuer, Thorsten Burmester: The occurrence of hemocyanin in Hexapoda. In: FEBS Journal . Volume 276, 2009, pp. 1930-1941. doi: 10.1111 / j.1742-4658.2009.06918.x
10. H. Watanabe, Hiroaki Noda, G. Tokuda, N. Lo: A cellulase gene of termite origin. In: Nature . 394, 1998, pp. 330-331.
11. Andreas Brune, Moriya Ohkuma: Role of the termite gut macrobiota in symbiotic digestion. In: David Edward Bignell (Ed.): Biology of Termites: A Modern Synthesis. 2010, chapter 16.
12. K. Nakashima et al .: Dual cellulose-digesting system of the wood-feeding termite, Coptotermes formosanus Shiraki. In: Insect Biochemistry and Molecular Biology. Volume 32, No. 7, 2002, pp. 777-784.
13. Michael M. Martin, Joan S. Martin: Cellulose digestion in the midgut of the fungus-growing termite Macrotermes natalensis: The role of acquired digestive enzymes. In: Science. Volume 199, No. 4336, 1978, pp. 1453-1455.
14. Hirofumi Watanabe et al: A cellulase gene of termite origin. In: Nature. Volume 394, No. 6691, 1998, pp. 330-331.
15. Lo, Nathan, Hirofumi Watanabe, Masahiro Sugimura: Evidence for the presence of a cellulase gene in the last common ancestor of bilaterian animals. In: Proceedings of the Royal Society of London. Series B: Biological Sciences. Volume 270, Suppl 1, 2003, pp. S69-S72.
16. ^ Michael S. Engel, David A. Grimaldi: New light shed on the oldest insect. In: Nature . Volume 427, 2004, pp. 627-630. doi: 10.1038 / nature02291
17. Carolin Haug & Joachim T. Haug (2017): The presumed oldest flying insect: more likely a myriapod? PeerJ 5: e3402 doi: 10.7717 / peerj.3402
18. Carsten Brauckmann, Brigitte Brauckmann, Elke Gröning: The stratigraphical position of the oldest known Pterygota (insecta. Carboniferous, Namurian). In: Annales de la Société Géologique de Belgique. 117 (Hommage à Maurice Streel) Fascicule 1, 1994, pp. 47-56.
19. ^ Robin J. Wootton: Reconstructing insect flight performance from fossil evidence. In: Acta zoologica cracoviensia. 46 (suppl. - Fossil Insects), 2003, pp. 89-99.
20. ^ RJ Wootton, J. Kulakova-Peck: Flight adaptations in Palaeozoic Palaeoptera (Insecta). In: Biological Reviews. Volume 75, 2000, pp. 129-167. doi: 10.1111 / j.1469-185X.1999.tb00043.x
21. ^ Paul Whalley: Insects and Cretaceous mass extinction. In: Nature. Volume 327, 1987, p. 562. doi: 10.1038 / 327562b0
22. ^ Conrad C. Labandeira, J. John Seposki Jr .: Insect diversity in the fossil record. In: Science . Volume 261, No. 5119, 1993, pp. 310-315.
23. Peter Ward, Conrad Labandeira, Michel Laurin, Robert A. Berner: Confirmation of Romer's Gap as a low oxygen interval constraining the timing of initial arthropod and vertebrate terrestrialization. In: PNAS . vol. 103 no. 45, 2006, pp. 16818-16822. doi: 10.1073 / pnas.0607824103
24. Jon F. Harrison, Alexander Kaiser, John M. Vanden Brooks: Atmospheric oxygen level and the evolution of insect body size. In: Proceedings of the Royal Society. Series B, 277, 2010, pp. 1937-1946. doi: 10.1098 / rspb.2010.0001
25. ^ FM Carpenter: Studies on North American Carboniferous insects. 1. The Protodonata. In: Psyche. Volume 67, 1960, pp. 98-110.
26. ^ William A. Shear, Jarmila Kulakova-Peck: The ecology of Paleozoic terrestrial arthropods: the fossil evidence. In: Canadian Journal of Zoology. Volume 68, 1990, pp. 1807-1834.
27. According to more recent finds, holometabolic insects have existed since the Upper Carboniferous, but were obviously very rare in the beginning: André Nel, Patrick Roques, Patricia Nel, Jakub Prokop, J. Sébastien Steyer: The earliest holometabolous insect from the Carboniferous: a “crucial” innovation with delayed success (Insecta Protomeropina Protomeropidae). In: Annales de la Société entomologique de France. (ns) 43 (3), 2007, pp. 349-355.
28. Olivier Béthoux: The earliest beetle identified. In: Journal of Paleontology. 6, No. 83, 2009, pp. 931-937. doi: 10.1666 / 08-158.1 .
29. Evgeny M. Zdobnov, Peer Bork: Quantification of insect genome divergence. In: Trends in Genetics . Vol. 23, No. 1, 2007, pp. 16-20. doi: 10.1016 / j.tig.2006.10.004
30. SL Cameron, KB Miller, CA D'Haese, MF Whiting, SC Barker: Mitochondrial genome data alone are not enough to unambiguously resolve the relationships of Entognatha, Insecta and Crustacea sensu lato (Arthropoda). In: Cladistics. Volume 20, 2004, pp. 534-557. doi: 10.1111 / j.1096-0031.2004.00040.x
31. J. Bitsch, C. Bitsch, T. Bourgoin, C. D'Haese: The phylogenetic position of early hexapod lineages: morphological data contradict molecular data. In: Systematic Entomology. Volume 29, 2004, pp. 433-440. doi: 10.1111 / j.0307-6970.2004.00261.x
32. James B. Whitfield, Karl M. Kjer: Ancient Rapid Radiations of Insects: Challenges for Phylogenetic Analysis. In: Annual Revue of Entomology. Volume 53, 2008, pp. 449-472. doi: 10.1146 / annurev.ento.53.103106.093304
33. KD Klass: Die Stammesgeschichte der Hexapoden: a critical discussion of recent data and hypotheses. In: Denisia. Volume 20, 2007, pp. 413-450. (PDF)
34. ^ RG Beutel, F. Friedrich, T. Hörnschemeyer, H. Pohl, F. Hünefeld, F. Beckmann, R. Meier, B. Misof, MF Whiting, L. Vilhelmsen: Morphological and molecular evidence converge upon a robust phylogeny of the megadiverse holometabola. In: Cladistics. Volume 27, 2011, pp. 341-355. doi: 10.1111 / j.1096-0031.2010.00338.x
35. Jerome C. Regier, Jeffrey W. Shultz, Andreas Zwick, April Hussey, Bernard Ball, Regina Wetzer, Joel W. Martin, Clifford W. Cunningham: Arthropod relationships revealed by phylogenomic analysis of nuclear protein-coding sequences. In: Nature. Volume 463, 2010, pp. 1079-1083. doi: 10.1038 / nature08742
36. Keisuke Ishiwata, Go Sasaki, Jiro Ogawa, Takashi Miyata, Zhi-Hui Sua: Phylogenetic relationships among insect orders based on three nuclear protein-coding gene sequences. In: Molecular Phylogenetics and Evolution. Volume 58, Issue 2, 2011, pp. 169-180. doi: 10.1016 / j.ympev.2010.11.001
37. ^ Robert B. Davis, Sandra L. Baldauf, Peter J. Mayhew: Many hexapod groups originated earlier and withstood extinction events better than previously realized: inferences from supertrees. In: Proceedings of the Royal Society London. Series B, 2010. online (Downloaded from rspb.royalsocietypublishing.org on August 18, 2011) doi: 10.1098 / rspb.2009.2299
38. Willi Hennig : The tribal history of the insects . Kramer, 1969.
39. Comprehensive presentation e.g. B. in: Jan Zrzavý: Four chapters about the monophyly of insect 'orders': A review of recent phylogenetic contributions. In: Acta Entomologica Musei Nationalis Pragae. Vol. 48 (2), 2008, pp. 217-232. (PDF)
40. Pentagon's new weapon - cyborg flies that are spies theguardian.com, March 15, 2006.
41. E. Youngsteadt, RC Henderson, AM Savage, AF Ernst, RR Dunn, SD Frank: Habitat and species identity, not diversity, predict the extent of refuse consumption by urban arthropods. In: Global Change Biology. 2014. (online before print) doi: 10.1111 / gcb.12791
42. ^ Francisco Sánchez-Bayo, Kris AG Wyckhuys: Worldwide decline of the entomofauna: A review of its drivers . In: Biological Conservation . tape 232 , 2019, pp. 8–27 , doi : 10.1016 / j.biocon.2019.01.020 (English).
 This version was added to the list of articles worth reading on January 6, 2006 .