Delicate Insects with atrophied mouth and small, short antennae; with four membranous wings having much minute cross-veining; the hinder pair very much smaller than the other pair, sometimes entirely absent: the body terminated by three or two very elongate slender tails. The earlier stages are passed through in water, and the individual then differs greatly in appearance from the winged Insect; the passage between the two forms is sudden; the creature in its first winged state is a subimago, which by shedding a delicate skin reveals the final form of the individual.
Fig. 273.—Ephemera danica, male, Britain.
The may-flies are well known—in literature—as the types of a brief and ineffective life. This supposed brevity relates solely to their existence in the winged form. In the earlier stages the may-fly is so unlike its subsequent self that it is not recognised as a may-fly by the uninitiated. The total life of the individual is really quite as long as that of most other Insects. The earlier stages and life-histories of these Insects are of great importance. The perfect Insects are so delicate and fragile that they shrivel much in drying, and are very difficult to preserve in a condition suitable for study.
The mouth of the imago is atrophied, the trophi scarcely existing as separate parts. Packard says that in Palingenia bilineata he could discover no certain traces of any of the mouth-parts, but in Leptophlebia cupida he found, as he thought, the rudiments of the maxillae and labium, though not of the mandibles. The antennae are always short, and consist of one or two thick basal joints succeeded by a delicate needle-like segment, which, though comparatively long, is not divided. The ocular organs are remarkable for their large size and complex development; they are always larger in the male than they are in the female. The compound eyes of the former sex are in certain species, e.g. Cloëon (Fig. 274), quite divided, so that each eye becomes a pair of organs of a different character; one part forms a pillar facetted at its summit, while the other part remains as a true eye placed on the side of the head; in front of these compound eyes there are three ocelli. Thus the Insect comes to have three different kinds of eyes, together seven in number.
Fig. 274.—Front of head of Cloëon, male. a, Pillared eye; b, sessile eye; c, ocellus.
The prothorax is small, the pronotum being, however, quite distinct. The mesothorax is very large; its notum forms by far the larger part of the upper surface of the thoracic region, the metathorax being small and different in structure, resembling in appearance a part of the abdomen, so that the hind wings look as if they were attached to a first abdominal segment. The mesosternum is also disproportionately large in comparison with the homologous piece preceding it, and with that following it. The pleural pieces are large, but their structure and disposition are only very imperfectly understood. The coxae are small and are widely separated, the anterior being, however, more elongate and approximate than the others. The other parts of the legs are slender; the number of joints in the tarsi varies from five to one. The legs throughout the family exhibit a considerable variety of structure, and the front pair in the males of some species are remarkably long. The abdomen is usually slender, and consists of ten segments; the terminal one bears three, or two, very long flexible appendages. The first dorsal plate of the abdomen is either wanting or is concealed to a considerable extent by the metanotum. The wings are peculiar; the anterior pair vary a great deal in their width, but are never very long in proportion to the width; the hind pair are always disproportionately small, and sometimes are quite wanting. The venation consists of a few, or of a moderate number, of delicate longitudinal veins that do not pursue a tortuous course, but frequently are gracefully curved, and form a system of approximately similar curves, most of the veins being of considerable length; close to the anterior margin of the wing there are two or three sub-parallel veins. Frequently there are very numerous fine, short cross-veinlets, but these vary greatly and may be entirely wanting.
Fig. 275.—Wings of Ephemera danica. (After Eaton.)