Its sluggish habits and marvellous camouflage make it very difficult to find: furthermore, it often buries itself in the gravel. Young specimens may be found in very fine gravel, but more mature nymphs occur on coarse gravel and boulders. It prefers pools and slow currents, but not stagnant water.

A curious trait is its preference for particular spots; thus, I have taken over twenty specimens from one small pool, whereas a protracted hunt over a hundred yards of adjacent stream-bed, containing many apparently precisely similar pools, has yielded a bare half-dozen.

The colour variation of this nymph is remarkable: it is sometimes brown, sometimes greyish-green, sometimes whitish, sometimes reddish, but the legs and venter show two clearly-defined types, referred to below as type A and type B.

Head—Small: held almost at right angles to the body: greyish-green, mottled with light brown.

Antennae—Short; twelve-jointed.

Eyes—Placed laterally; large; olive green.

Labrum (Text-Fig. 50a)—Nearly twice as broad as it is long. Anterior edge nearly straight—only slightly curved medianly; profusely covered with inwardly-pointing spines, their tips curved inwards; this fringe extends along the lateral edges three-quarters of the way towards their posterior angles. These angles are somewhat oblique and very dark in colour. There are additional incurved spines on the anterior region of the dorsal and ventral surfaces.

Mandible (Text-Fig. 50)—Both outer (o.c.) and inner (i.c.) canines have four teeth, but so placed that the examination of a slided specimen seldom reveals more than three. Prostheca (p) narrow and shorter than the canines, with rounded, non-chitinised tip and covered with fine, short hair. Placed interiorly and curving inwards is a brush of long light brown hair. Molar surface (m) with about 10—the number varies—parallel ridges with serrated edges.

Maxilla (Text-Fig. 51)—Palp three-jointed; somewhat longer than the galea-lacinia: the basal joint is the broadest, the distal one the narrowest: they are all about the same length, the middle joint being very slightly shorter than the two others. There are a few spines on the edge of each joint and a number situated apically on the distal one. The galea-lacinia narrows anteriorly to a point; from the places where it begins to narrow to the apex, it is fringed with hair.

Hypopharynx (Text-Fig. 52a)—Median piece projecting slightly beyond the superlinguae anteriorly, lateral angles rounded, lateral margins somewhat gibbous. The surface appears corrugated but unhaired. Superlinguae, each about half as wide as the median piece, becoming narrower basally. The anterior and interior margins are heavily fringed with hair.

Labium (Text-Fig. 52)—Palp three-segmented: basal segment broadest and longest, very dingy and covered with short incurved spines: second segment with a few spines: distal segment much shorter and somewhat narrower than the others, its apex pointed and covered with spines. Paraglossae falcate with numerous spines anteriorly. Glossae a little but not very much smaller than the paraglossae,

their inner edges somewhat concave, touching each other distally; their outer edges are slightly convex, the curvature increasing anteriorly; the inner anterior angles are quadrangular with the extreme corners, only, rounded. The glossae bear a number of marginal spines.

Thorax—Greyish-green, mottled with light brown. Pronotum narrow: mesonotum very broad, V-shaped, almost covering metano-

Nymph of Oniscigaster intermedius.
Fig. 50.—Left mandible and labrum. × 30.
Fig. 51.—Right maxilla. × 30.
Fig. 52.—Labium and hypopharynx. × 30.
Fig. 53.—Dorsal crest (2nd to 7th segments). × 20.

tum Wingpads small, greyish-green, covering the first two abdominal segments only.

Legs (Text-Fig. 48)—Short; robust. Order of length, 3, 2, 1: this is accounted for by the differing length of femur, the tarsi and tibiae not varying in length. The tarsus is longer than the tibia. The claw is about half the length of the tarsus, swollen at the base, and pointed and hooked apically: it is untoothed. There is a row of spines on the ventral edges of the tarsus and tibia.

Eaton in his plate 51, vol. 3, A Revisional Monograph of Recent Ephemeridae, Trans. Linn. Soc., Lond., describing the nymph of Oniscigaster wakefieldi, shows a row of spines on the dorsal edge of the leg. Either this is an error, or it offers a means of distinguishing the two species in their nymphal stages, and apart from this difference, his description appears to tally, so far as it goes, with the species described in this paper.

Colouration—Type A—Femora greenish-white, with brown bar two-thirds of the way towards the distal end. Tibiae white, with tinge of green. Tarsi white, with dark greenish-grey mark at proximal end and dark brownish-grey mark distally.

Type B—Femora yellowish-white, with brown bar two-thirds of the way towards distal end. Tibiae light yellow, with brown mark at each end. Tarsi light brown, darker at each end. In both types the claws are stout, brown and strongly curved.

Abdomen—Type A—Greyish-green with brown markings. Type B—Light brown, mottled with light yellow. The first nine segments have whitish pleurae laterally flanged, the posterior lateral angles of which curve backwards and terminate in sharp, black points. The tenth segment has no flanges, but the posterior edge, which is dark-rimmed, slopes back medianly, terminating in a rounded point; there is a pair of medianly placed brown dots on this segment. The first nine segments are keeled longitudinally along the median line, and produced into sharp points, projecting over the next (posterior) segment (Text-Fig. 53).

Venter (Text-Fig. 49) Type A—Ground colour dingy white with maroon markings, placed laterally on each of the 2nd to 9th segments and a thin band joining them running along the anterior edge. There are also two median markings of this colour, one quarter of the way from the anterior edge, and two smaller dots closer to the median line and half way towards the posterior edge. The flanges are dingy white and the markings on each segment somewhat resemble a horned owl.

Type B—Ground colour light chocolate, lateral flanges flavescent: the second to seventh segments have a prominent whitish W mark medianly (eighth and ninth segments faintly so), with two paired brown markings inside the apices of the arms of the W and two small brown dots within the lower part. Faint indefinitely-shaped brown areas jut out into the flanges.

In both types the tenth segment is very dingy and emarginate posteriorly.

Caudal setae—Short; white, except for a small area towards the tip, which is black. Median one with a long fringe of hair on both sides, outer ones fringed on inside only: there are circles of minute