Enallagma annexum
| Enallagma annexum | ||||||||||||
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Enallagma annexum , male |
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| Scientific name | ||||||||||||
| Enallagma annexum | ||||||||||||
| ( Hagen , 1861) |
Enallagma annexum is a species of dragonfly from the family of the common dragonfly (Coenagrionidae), which is distributed in large parts of Canada and North America and can become very common in northern latitudes. Swampy open lakes and ponds, often moor lakes, are settled. Enallagma annexum is a typical representative of the numerous group of blue-colored cup maids and is confusingly similar to the other members, especially the Enallagma boreale, which is almost identical in ecological requirements. Only recently has it been recognized that the nearctic population of Enallagma annexum , previously identified as the common mugwort ( Enallagma cyathigerum ), is very similar to it, but is genetically separate from it. Since then it has been run as the Enallagma annexum . Thespecies was first described in 1861 by Hermann August Hagen .
features
Enallagma annexum is a typical slender dragonfly with a body length of 29 to 40 millimeters and a length of the rear wing of 18 to 20 millimeters. The males have a light blue basic color with large teardrop-shaped and also blue postocular spots on the body-facing side of the compound eyes . These are aligned with one another with the tip and connected to one another via a line. The eyes are dark on top, so that they appear to be covered by a cap. The median strip on the upper side of the front body ( thorax ) is noticeably wide, while the lateral humeral strips are narrow. On the underlying light blue thorax sides there are two thorax lateral stripes, the upper one is only very rudimentary or not at all, so that the thorax side surface appears as if it were not drawn. The elongated abdomen is provided with a typical slender vial mark. The lower end of the second segment has a broad black horizontal line, often curved in the shape of a crescent. On the other segments three to seven there are caudally located, wider black rings, so that segments six and seven appear almost entirely black. The eighth and ninth abdominal segments are then completely blue, the tenth black on top again.
The populations in Alaska and Canada west of the Rocky Mountains appear darker because of the more extensive black markings. Your humeral stripes are wider and the black drawing of the abdomen is much more pronounced. There is an extensive spot on the second abdominal segment, and much of the abdominal segments four and five are black. The populations on the Pacific coast are also on average darker than those inland.
The abdomen of the females is more powerfully built than that of the males and the upper side is predominantly dark in color, the abdominal segments only show a light ring at the front edge of segments three to eight. In front of the ovary, on the eighth abdominal segment, there is a protruding thorn. The light basic color of the dragonfly is most pronounced on segment eight, this can sometimes even be completely without black markings. Segments nine and ten are black. The eyes of the females are divided horizontally in color, the upper part is dark, the lower light brown, on the lower light part there is usually a dark horizontal stripe. The chest drawing corresponds to that of the male. As is not uncommon in the subfamily of the Ischnurinae , the females come in different color variants. There is an androchromic form, colored like the males, and a brown, heterochromic form.
Similar species
A relatively recent evolutionary split has led to a large number of species, some of which are very similar and difficult to distinguish, in North America. However, the cup youngsters are often formed from only one species, and once determined, it can be assumed that all specimens of this population only belong to this species. Preferences for different habitat types, which at first glance appear very similar, seem to separate them, even if these mechanisms are not yet fully understood.
Enallagma annexum can not be distinguished from Enallagma vernale without an enlarged examination of the male abdominal appendages, which differ only in fine details. In E. vernale , each cercus has a button-like thickening at its distal end, at the foot of which there is a small black tooth next to a small depression. E. annexum lacks these characteristics, but intermediate types that share characteristics of both species appear again and again , either through natural variation or through hybridization . E. vernale is found relatively often in waters with fish stock, E. annexum seems to avoid them, or cannot reproduce here.
Enallagma boreale is also very difficult to distinguish from Enallagma annexum . In the former, a lip-shaped elevation can be seen in the enlargement of the male abdominal appendages, in E. boreale this is located on the inside of the cerci. Enallagma annexum is also very similar to Enallagma civile , but this species has longer upper abdomen appendages than lower ones, exactly the opposite of E. annexum . In E. civile , the drawing on the upper side of the second abdominal segment is larger and touches the rear end of the segment, this mark separates E. civile from E. annexum and E. boreale in areas where the distribution areas of these dragonflies overlap. Enallagma clausum is also very difficult to distinguish from Enallagma annexum , where the markings on the fifth abdominal segment are usually less pronounced. Also Enallagma hageni and Marsh Bluet similar enallagma annexum , but are smaller overall and have smaller Postokularflecken, this may be particularly in determining the females of concern. In most of the areas where the distribution areas of dragonflies overlap, the horizontal line on the second abdominal segment distinguishes E. annexum from the two smaller species, which have larger spots here.
The females of Enallagma annexum , Enallagma clausum and Enallagma boreale are also very similar, a determination can be made by comparing the prothorax and its typical shape of the posterior margin. This is clearly developed in E. annexum , while in E. boreale it fuses with the thorax. The shape of the prothorax of E. clausum is more like that of E. annexum , but the front edge is constricted and there are two small pits on the top. In females of Enallagma recurvatum , the eighth abdominal segment is also almost completely without drawing, but these are significantly smaller. This distinctly bright eighth abdominal segment does not occur in the females of other similar cup maids.
distribution
Enallagma annexum is distributed throughout western North America, from Alaska to northern Baja California , in eastern North America from Newfoundland to West Virginia .
Way of life
Enallagma annexum can become very common in northern latitudes. Swampy open lakes and ponds, often moor lakes, are settled. In eastern North America, it is a typical fish-free dragonfly. In the west, E. annexum can also be found on slow-flowing rivers and is not bound to fish-free waters, in California it is likely that only rivers are populated.
The males are - typical for the cup maid - to be found in large numbers near the water, where they fly low over the open water in large numbers or rest on the edge of dense vegetation structures. Mating usually takes place in sunny clearings close to the water, with copulation lasting 10 to 27 minutes. Eggs are laid in floating vegetation that does not protrude from the water; vascular plants are preferred over algae. Many possible egg-laying places are checked beforehand. The eggs are usually laid in tandem, the pair may separate and the female dives upside down to continue underwater, the diving time can be up to 90 minutes. The male mostly guards his partner above the water surface, but often gives up after a while. There is hybridization with Enallagma vernale .
The flight time of the adults differs with the distribution. In Yukon it is between June and August, in California it is from April to November. In eastern North America it begins in May and extends on the southern edge of the range, in Ohio , until July, on the northern edge, in Nova Scotia , until October.
swell
literature
- Dennis Paulson: Dragonflies and Damselflies of the East, Princeton Field Guides , Princeton University Press, New Jersey 2011, ISBN 978-0-691-12283-0 .
- Dennis Paulson: Dragonflies and Damselflies of the West, Princeton Field Guides , Princeton University Press, New Jersey 2000, ISBN 978-0-691-12281-6 .
Individual evidence
- ↑ a b c d e f g h i Dennis Paulson: Northern Bluet. In Dragonflies and Damselflies of the East , pp. 95-97.
- ^ A b c Dennis Paulson: Northern Bluet. In Dragonflies and Damselflies of the West , pp. 87-88.
- ^ Jill Silsby: Subfamily Ischnurinae (Blue-tailed Damselflies). In Dragonflies of the World . Smithsonian, Washington 2001, ISBN 1-560-98959-9 , pp. 110-112.
- ^ A b Dennis Paulson: American Bluets. In Dragonflies and Damselflies of the East , p. 85.
- ^ A b c Dennis Paulson: Vernal Bluet. In Dragonflies and Damselflies of the East , pp. 97-98.
Web links
- Photos of Enallagma annexum (formerly E. cyathigerum ) - Northern Bluet on odonata.bogfoot.net, with pictures