468 MODERN CLASSIFICATION OF INSECTS, 
having occasionally a minute rudimental joint at their base; the rostrum 
is of moderate length, the third (or the penultimate) joint being consi- 
derably longer than the others ; the upper lip is distinct, long, and much 
narrowed in its apical part ; the legs vary considerably in their struc- 
ture in the different genera; the anterior, however, are in none rap- 
torial, although their peculiar position in Gerris (fig. 120. 2. Gerris 
Paludum) renders them evidently serviceable in retaining their prey ; 
the tarsi are short and 2-jointed; occasionally, however, 3-jointed, as 
in the fore tarsi of Velia (fig. 120. 5.), in which certain minute mem- 
branous retractile lobes have been observed by M. Doyére (Ann. Soc. 
Ent. de France, 1837, and Crochard ed. FR. An. pl. 92.). The ungues 
are often inserted in a notch before the tip of the tarsus. 
Of these insects Gerris ( fig. 120. 2.) is by far the most active, the 
species skimming along the surface of the water with great velocity, 
and turning about with the greatest ease ; for this purpose, the form 
of their bodies, nearly resembling that of a London wherry, is admir- 
ably fitted. Mr. Curtis observes, that “these insects, by rowing with 
their posterior feet, glide over the water :”’ but this is inaccurate ; the 
hind feet act conjointly as a rudder, whilst the longer middle feet, placed 
at the middle of the sides of the body, are used somewhat as oars * : 
they are not, however, dipped into, but merely brush along the sur- 
face of the water. Velia has a similar, although slower action, having 
much shorter feet; but Hydrometra (fig. 120. 6.) merely creeps 
slowly upon the surface, the body of the insect being considerably 
elevated ; hence it is mostly found amongst the low plants growing 
out of and at the side of water. 
Many of these insects are subject to a remarkable diversity in their 
development in the imago state; many of them are found in a com- 
pletely apterous state, although in all other respects they are fully de- 
veloped and full-sized, and are often found coupled together in this 
state. By Geoffroy, who appears first to have noticed this fact, they 
were considered as larva, possessing procreative powers ; whilst Kirby 
and Spence regarded them as pupe (Introd. to Ent. vol. iv. p. 570. 
“ Inter pupas Orthopterorum et Hemipterorum coitus interdum lo- 
cum habet, quod maturiorem organizationem in his analogis quam in 
aliis insectis probat.”). Others, again (De Geer, Dufour, Curtis, &c.), 
* Tn the singular Gerris laticauda Hardw. (Linn. Trans. vol. xiv. pl. 6. fig. 1, 2. ; 
and Griffith, An. K. Ins. pl. 93. fig. 2.), the middle femora have a dense coating of 
hairs, and in Velia the middle tibia are more sparingly furnished in the same man- 
ner, thus offering a broader surface for opposing action. 
