BEETLES. 391 



vittatus is the largest species from the United States, where it is common in the 

 Atlantic region. It is about 0.55 of an inch long, and the elytra have rounded tips. 



The larvae of Gyrinid® respire by means of ciliate gills and a closed tracheal 

 system. These gills are situated one on each side of every abdominal segment, and an 

 additional pair upon each side of the anus, — in all, ten pairs of gills. These ciliate 

 branchiae serve as swimming organs, enabling the larvas to swim rapidly. ISTotwith- 

 standing their active aquatic life these larvae are subject to parasitism by three or four 

 species of Hymenoptera. 



The flattened water-beetles belonging to the family Dttiscid^ live in the water as 

 do the Hydrophilidae, rather than upon it like the Gyrinidae, from all of which insects 

 they are readily distinguished by their filiform eleven-jointed an- 

 tennae. The Dytiscidae have but two eyes, their metasternum has 

 no antecoxal piece, and is prolonged in a triangular process pos- 

 teriorly, their abdomen has six segments. The legs of these beetles, 

 especially the posterior pair, are oar-shaped and clothed with long 

 hair, being consequently well fitted for natation ; but these insects 

 are not confined to swimming as a mode of locomotion, for they are 

 provided with serviceable wings, and during the night fly from pond 

 to pond. These beetles are apparently guided to water by sight, 

 Fig. 466. —iTpdmporus for they often fly against green-houses, the glass of which they mis- 

 take for the surface of water. Many Dytiscidae emit between the 

 head and prothorax, a milky secretion, and between the mesothorax and metathorax a 

 yellowish fluid ; the function of both these odorless fluids is not determined with cer- 

 tainty. At each side of the anal end of the intestine are two glands which furnish a 

 strong odorous, acid fluid, said to contain butyric acid ; these anal glands, which are 

 not rare in adephagous Coleoptera, are defensive in function. 

 Many species of Dytiscidae stridulate ; that is, produce more or 

 less musical sounds, both under water and in air. Species of 

 Acilius, Dytiscus, and Colymbetes produce sound by rubbing 

 the abdominal segments upon the elytra ; the males of Cyhister, 

 by action of the posterior femora upon a corrugated spot behind 

 the hind coxae ; and Felobius, a genus placed by some authors 

 in another family, by friction of the stout margin of the wings 

 against the under side of the elytra. The anterior and often 

 the middle tarsi of males, in certain genera of this family, have 

 a part of their joints wijJened, and provided with pedunculate 

 suckers beneath. These suckers enable the males to adhere 

 firmly to the females during copulation. In the males of Bytis- 

 cus and of some allied genera the three basal joints of the an- via.tsj.—Dytisaus 

 tenor tarsi, which are the ones most strikingly modified, are so 



widely expanded as to form unitedly a saucer-shaped disc, which bears on its under sur- 

 face two large and numerous small chitinous suckers mounted on stems. These some- 

 what toadstool-formed suckers are altered tarsal hairs. The females of some species of 

 Dytiscidae exhibit an interesting dimorphism in that some of the individuals have the 

 elytra striate, while others of the same species have them smooth. Different species of 

 Dytiscus iiave been known to copulate with each other, and Kraatz has published a- 

 notice of a bastard between two species of this genus. Dytiscid^ are rarely found in 

 salt water, and their occurrence there is apparently accidental and temporary. Species 



