The Ephemeridœ are insects with a long, soft, 10-jointed abdomen, furnished at its hinder end with either two or three many-jointed setaceous or filiform tails (caudal setæ). The body is smooth and glabrous. The head is free, with atrophied mouth - organs and carinated epistoma; short subulate antennae, composed of 2 or 3 short stout joints, succeeded by a many-jointed setaceous awn, three ocelli, and large oculi (compound eyes). Thorax robust, mesothorax predominant, sternum well developed; fore wings ample, erect or spreading in repose, slightly plaited lengthwise; legs slender, femora strong, the fore coxæ somewhat distant from the others. The abdomen in the male armed with a pair of claspers (forceps) placed ventrally at the extremity of the penultimate segment; the vasa deferentia have each a separate intromittent organ, situated at the ventral joining of the ninth and tenth segments.
Peculiarities in structural detail are often noticeable in both or one of the sexes, and are chiefly presented by the ocelli, wings, legs, and caudal setæ; and in the male by the ocelli and forceps. The ocelli are always much larger in the male than in the female, and are divided into two parts transversely; the upper portion has larger facets than the lower, and is sometimes coloured differently.
The fore wings are usually trilateral, ample and rounded off at the extremities; they are relatively longer in the female. The hind wings in some of the genera are not developed; in others they are very minute; and generally they are not very large. Their usual form is triangular-ovate, or oblong-ovate, with a salient prominence in front, either close to the wing-roots or midway towards the apex, in. which case the prominence is sometimes followed by a deep depression. Their neuration is fairly plentiful. The inner margin of the fore wing and the anterior margin of the hind wing hitch together automatically to a larger or smaller extent when the wings are spread open. The wing-membrane is usually glassy or iridescent in the adult. Wing-neuration in the Ephemeridœ is less complicated than it appears to be, and when difficulty is experienced in ascertaining the homologies
of the nervures it is likely to be occasioned by the suppression of some of them. Unstable in minutiæ, so closely is the essential plan of neuration adhered to by nearly related mayflies that the general facies of the wing is an important aid to classification. The nervures are numbered in the diagrams as follows: 1, the costa, coincident with the anterior margin of the wing; 2, the subcosta; 3, the radius; 4, the sector; 5, the cubitus; 6, the præbrachial; 7, the pobracbial; 8, the anal; 91, 92, &c., axillary nervures; 10, the sutural, coincident with the inner margin.
The nervures of the fore wing arrange themselves in three groups. The first—consisting of the costa (1), the subcosta (2), and the radius (3)—communicates directly with the thorax; the second—containing the sector (4), the cubitus (5), the præbrachial (6), and the pobrachial (7)—is either annexed to the first group, or terminates in the wing-membrane adjacent to it, close to the base of the wing; the third group—consisting of the anal (8) and the axillary nervures (91, 92, &c.)—is associated with the prominent curved or angulated crease in the membrane which forms the boundary of a depression posterior to the great cross-vein and close to the wing-roots. By careful inspection of the third group of nervures, observing especially the disposition of the proximal extremities of the main nervures along the prominent curved fold of the membrane, the form of the area contained by the first axillary nervure and the inner margin of the wing, or of the area enclosed by the first and second axillary nervures, and lastly by the general aspect of the adventitious and other nervures, the approximate affinities of Ephemeridœ to one another can be ascertained very easily. Cross-veinlets are generally of small account in classification.
In the nervures of the hind wing the cubitus (6) is transferred from the second group, and is annexed to the radius (3), the sector and other adventitious nervures either remaining apart from both or forming a union with either of them. The anal nervure (8) is transferred to the second group. The axillary nervures forming the third group generally occupy a very limited space.
The legs present great differences, some sexual, some generic. The fore legs are always longer in the male than in the female, and are usually longer than either of the hinder pairs. The fore tarsus is often as long as the tibia; in the male frequently much longer. The number of tarsal joints is 5, or 4. The ungues of the fore tarsus are sometimes alike in size and form; often unlike.
The forceps of the male are 2-, 3-, or 4-jointed, with the basal joint, or the next, longest. In some genera they afford good distinctive characters of species.
Much diversity is exhibited in the number and relative proportions of the caudal setæ.
The term “nymph” is used to denote all the subaqueous stages in the development of the young after hatching. In general form they resemble the adult. The tracheal branchiæ are movable, membranaceous, or filamentose appendages to the integuments, enclosing branching tracheæ. The term “subimago” is used to denote the penultimate stage in the life of such of the Ephemeridœ as moult once, after direct respiration through the stigmata has been established, and the wings have become fully expanded. The chief points by which this stage can be distinguished from the adult are—the dulness of the integuments, particularly of the wings; the ciliolate terminal margin of the wings in many genera; the shortness of the fore legs; the greater hairiness and shortness of the caudal setæ; the less protuberant and less highly coloured ocelli; and in the male the marked shortness and stoutness of the limbs of the forceps.
The above account is condensed from Eaton's Monograph.
In the “Revisional Monograph of Recent Ephemeridœ, or Mayflies,” by the Rev. A. E. Eaton, M.A., published in the “Transactions of the Linnæan Society,” London, 1888, the following genera are given as represented in New Zealand:—
Ephemera (1 species).—Undescribed.
Atalophlebia (3 species).—A. dentata, A. nodularis, A. scita.
Coloburus (1 species).—C. humeralis.
Siphlurus (1 species).—Doubtful.
Oniscigaster (1 species).—O. wakefieldi.
Chirotonetes (?) (1 species).—C. ornatus.
Thus it will be seen that six species are described, but of these the nymph stage of only one (Oniscigaster wakefieldi) is known.
During the present summer I succeeded in rearing a number of insects of two species of Atalophlebia from the nymph stage, and am consequently able to describe all three stages.