GTG I'A.MII.Y XXX 111. PARXID.1<:. 



acinatic i)laiits. They feed upon (IccMiiiiposin^' inatlcM- in the water 

 and tliiis form one of Ihc aucncies whlcii nature has [provided for 

 l)nrifyinji' streams. The less are not titted for swinniiing. l)ut the 

 fifth or hist joint of the tarsi is longer than the other four united, 

 and is armed with long, simple claws, whence the common name 

 above given. This structure of the tarsus is the principal distin- 

 guishing character of the family and enables its members to grasp 

 firmly objects resting in strong curn^nts of water. By taking an 

 old liml) or rough stone from the water at a siiitahle locality and 

 placing it in the sun, the insects will move as the water dries, 

 though at first nothing can be seen of them, so perfectly do they 

 resemble the surface on which they rest. The surface of tlie bod.y is 

 clothed with fine silken hairs whicli have the property of repelling 

 or shedding water and enable the insect to surround itself with a 

 film or globule of air while clinging to objects beneath the water. 



The larva of our largest species, PsepJienus lerontci, resembles in 

 appearance a trilobite, being flat and nearly circular in outline, and 

 measuring about 8 mm. in length. It is found clinging to stones in 

 rapid water or in muck near springs or ponds. The larvfe of the 

 genera Elmis and Stenelmis are said to be similar in form, except 

 that the segments are not united to the margin, which thus appears 

 notched or incised. 



Among the more distinctive characters of the family are the 

 usually retractile head, with distinct labrum and small mandibles; 

 prosternum distinct in front of the coxa", usually elongate behind 

 and forming a process received into a definite cavity in the meso- 

 sternum ; front coxal cavities widely open ; hind coxa3 transverse and 

 usually dilated into a plate, which j)artly ]>rotects the thighs; elytra 

 entire; abdomen usually with five venti'al segmenls, llie front ones 

 coiniate or firmly united. 



Nearly 400 species ol' Hie raiiiily are known, abonl 50 of whicli 

 occur in Xorlli Ainerica. The principal lileralure Ireating of these 

 is as r(»llows: 



/yrToy/Zr. -"S\'n()i)sis of llie Parnida' of the I'niled States," in 



Proc. Phil. Acad. Nat. Sci.. VT. 1852, 41-44. 

 IJov)). — "Syno])sis of the I'arnida' of the TTiiited States," hi 



Trans. Amer. Enl. Soc.. Til. 1870, 29-42. 



The North American species are distributed among three sub- 

 families, all of Avhich are rey>resented in Indiana. 



