390 



NATURAL HISTORY OF ARTHROPODS. 



Fig. 463. — UudroiihUus 

 iriangutari.s. 



shining black species, tuund in the northeastern United Slates. The form is nearly 

 hemispherical, the length from 0.8 to 0.4 of an inch. 



The genus JL/drophihis contains the largest species of this family. They differ 

 from Hijdrobiaii and genera associated with it in having compressed tarsi, and in hav- 

 ing the metasternuni prolonged posteriorly into a long sjiine. 

 H. trktuguluris, a common North American species, is 1.5 inches 

 long, and shining black in color. The egg-case of this species 

 has been carefully described by Dr. C. V. Riley. It consists 

 of three parts, a floater surmounted by a horn-shaped process, 

 au egg-case projjer, attached to the basal end of the floater, and 

 an outer bag or covering. The larvae remain a day or two in 

 the outer bag after they hatch ; and Dr. Riley concludes " that 

 the curious contrivance in JL/drophilKs is intended not only to 

 secure an anijile supjily of air to tlie eggs and to protect them, 

 but also to ]irotect the newly hatclied young from their nu- 

 merous enemies until their jaws have strengthened and they are 

 better able to begin the struggle for existence." The eggs are 

 white, cylindroid, and aViout 0.15 of an inch long and .04 of an 

 inch in diameter. In Euro|ie the corresponding large s]iecies of 

 this genus is II. iiircas, both se.xes of which are figured above, 

 with nest and larva. The sexes of these insects are easily dis- 

 tinguished, because the male has the anterior tarsi much en- 

 lai'ged, to form an organ, common among water beetles, by which 

 he clings firmly to the female during copulation. Notwithstanding the rejjuted phyto- 

 phagous habits of the imago of H. ji ictus, this beetle sometimes captures and eats Tri- 

 t07i punctatus, when confined in an aquarium. 



Nearly related to Ili^/drophilus, but having a short metasternal spine, is Hydro- 

 charis. II. obtasatus, an oval black sj)ecies, about 0.7 of an inch long, 

 is common in the eastern United St.ates. Several smaller species are 

 united in the genus IVopliiternus, of which T. ijlahtr is an abundant 

 species in New England. 



The remaining families of lieetles are often termed Adephaga, and 

 are characterized by Drs. Le Conte and Horn as having the first three 

 ventral segments connate; the first divided by the hind coxal cavities, 

 so tliat the sides are separated from the \ery small medial part. I'lie 

 Adephaga are pre-eminently predaceous Coleojiti'ra, ;ihhough, as v.'ill fig. 464. — Ti-opi- 

 be seen "later, a few jjartake of vegetable food. "" st.mus giahcv. 



The Gyrinid.e includes a small number of Coleo])tera which swim rapidly about in 

 groups upon the water, unless distui-bed, when they dive beneath the sur- 

 face. They are noticeable for their having a pair of eyes upon the upper 

 surface of the head, with which to look out into the air, and a pair u])on llie 

 inider side for sight under water. Their flattened oval form and bluish- 

 black color, together with their four eyes, serve to distinguish them 

 from all other Coleoptera. Wiien seized they emit an odorous milky 

 fluid. In Gi/rinus the scutellum is distinct, while in Uineutus it is 

 wanting. The only other North .\niei-ican genus is Gi/retes, oi which a 

 species is found in Illinois, Ai'izona, and Texas, and which differs from Diiteutus \\\ 

 having the last ventral segment of the abdomen elongated and conical. Uijieutus 



Fig. 465.— /JiiiM/- 

 tus vittatHS. 



