390 



NATURAL HISTORY OF ARTHROPODS. 



FlO.463. — a/( 



triangularis. 



shining black species, found in the northeastern United States. The form is nearly 

 hemispherical, the length from 0.3 to 0.4 of an inch. 



The genus Hydrophilus contains the largest species of this family. They differ 

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

 ing the metasternum prolonged posteriorly into a long spine. 

 H. triangularis, 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, 

 an egg-case proper, attached to the basal end of the floater, and 

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

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

 the curious contrivance in Hydrophilus is intended not only to 

 secure an ample supply of air to the eggs and to protect them, 

 but also to protect the newly hatched 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 about 0.15 of an inch long and .04 of an 

 inch in diameter. In Europe the corresponding large species of 

 this genus is H. piceus, both sexes of which are figured above, 

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

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

 larged, to form an organ, Common among water beetles, by which 

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

 phagous habits of the imago of JI. piceus, this beetle sometimes captures and eats Tri- 

 ton punctatics, when confined in an aquarium. 



Nearly related to Hydrophilus, but having a short metasternal spine, is Hydro- 

 charis. H. obtusatus, an oval black species, about 0.7 of an inch long, 

 is common in the eastern United States. Several smaller species are 

 united in the genus IVopistemus, of wliich T. glaber is an abundant 

 species in New England. 



The remaining families of beetles 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 that the sides are separated from the very small medial part. The 

 Adej)haga are pre-eminently predaceous Coleoptera, although, as will 

 be seen later, a few partake of vegetable food. 



The Gyeinid^ includes a small number of Coleoptera which swim rapidly about in 

 groups upon the water, unless disturbed, 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 wliich to look out into the air, and a pair upon the 

 under 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. When seized they emit an odorous milky 

 fluid. In Gyrinus the scutellum is distinct, while in Bineutus it is 

 wanting. The only other North American genus is Gyretes, of which a 

 species is found in Illinois, Arizona, and Texas, and which differs from Dineutiis in 

 having the last \entral segment of the abdomen elongated and conical. Dineutus 



Fig. 464. — TropU 

 siemus glaber. 



Fig. 465.— Z)iM 

 tus viitaius. 



