Insect external structure
Insect internal structure
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Insect identification > Ephemerida
The Ephemerida, May flies or shad flies as they are often called, are insects of medium or small size. The adults have delicate bodies and gauzy, fragile wings, the latter usually with many cross veins. The forewings are much larger than the hind ones, which in some cases are absent, and the former are in general rather strongly triangular in outline. When at rest they are held vertically above the body.
At the end of the abdomen two or three long threads, each composed of many segments and often called caudal filaments, are usually present, the lateral ones being cerci corresponding to those in the Thysanura.
The mouth-parts of the adult May fly are of the chewing type, but so poorly developed that it is doubtful if they are ever made use of. In some cases they are even rudimentary. The reproductive organs differ from those in all the other groups, the ducts being not united on the middle line below, but opening separately to the outside-apparently the retention of a very primitive condition. The early stages are passed in the water, the nymphs breathing-at least after the first few molts by tracheal gills. These are delicate, usually wing-like in form, and are outgrowths of the body wall. Into them pass tracheal trunks which branch again and again so that only their own walls and those of the gill itself separate the air in the tracheae from that in the water outside, and so thin are these layers that the oxygen in the water can pass through them into the tracheae, and carbon dioxid gas pass out.
These insects add to their list of peculiarities also the fact that, after becoming full grown and being able to fly, they molt once more, even a thin layer over the final wings being shed.
From these statements the group may be characterized thus:
Insects having as adults delicate bodies and usually four wings, the front pair much larger than the others (which are sometimes absent), and generally with many cross veins; end of the abdomen with two or three long caudal filaments composed of many segments; reproductive organs with two openings to the exterior; mouth-parts of the chewing type but practically rudimentary; nymphs living in water and with an incomplete metamorphosis, the final molt coming after the wings have become fully developed.