BEETLES. 



391 



Flo. -JCti. — Hjiilrojivru 

 (iriscoatriatua. 



clttutus is the largest species from tlie I'uited States, where it is common in the 

 Atlantic region. It is al)out <).:');') of an inch long, and the elytra have roumled ti[>s. 



The larv;i3 of Gyi'inid;c resjiire I>y means of ciliate gills and a closed tracheal 

 system. These gills are situated one on each side of every alidominal segment, ami an 

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

 branchiai serve as swimming organs, enabling the larvie to swim rajiidly. Notwith- 

 standing their active aquatic lifi' these larvie are subject to parasitism by three or four 

 species of Hymenoptera. 



The flattened water-fieetles beloii<4ing to the family Dytisi iijj: live in the water as 

 do the Hydrophilidre, rather than upon it like the Gyrinidie, from all of which insects 

 they a.re readily distinguished l)y their filiform eleven-jointed an- 

 tenn;v. The Dytiscida- ha\ e but two eyes, their metasternum has 

 no antecoxal ]>iece, and is prolonged in a triangular process pos- 

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

 especialh' the postei'ior pair, are oar-shaped and clothed with long 

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

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

 pro\ idecl with serviceable wings, and during the night fly from pond 

 to pon<l. These beetles ai'e apparently guided to water by sight, 

 for they often fly against green-luuises, the glass of which they mis- 

 take for the surface of water. Many Dytiscida; emit between the 

 head and prothorax, a milky secretion, arid 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 defi'usive in function. 

 3Iany species of Dytiscidie stridulate ; thai is, produce more or 

 less musical siiunds, both un<ler water and in air. Species of 

 Aciliii.i, />i/ti.sc>i{<, and f/(i/i/iii//cf' s produce sound b\ rubbini;' 

 the aljdominal segments upon the elytra; the males of ('i/lilMei\ 

 \>\ action of tlie ]iosteri<.)r femora upon a corru!.;;ited spot behind 

 the hind (Mixa- ; and /'t7(;////^s, a geims placi'il liv some authors 

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

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

 the middle tarsi of nniles, in certain genera of this famih', have 

 :i jiart of their joints widened, and provided with ]ieduiiculatt 

 suckers l)eiieath. These suckers enable thi' males to adhere 

 firmly to the females during copulation. In the males of TJi/fi.s- 

 cun and of sonu' allied genera the three liasal joints of the an- 

 terior tarsi, which are the oiu-s most strikingly nioditieil, are so 

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

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

 what toadstool-formed suckers are altered tarsal hairs. The females of some s]ieciesof 

 Dytiscida' exhibit an interesting dimorphism in that some of the imliviiluals have the 

 elytra striate, while others of the same sjiecies h.ave them snnjoth. Different species of 

 Dytiscus have been known to i'o|mIate with each other, and Kraatz has ]>iiblished a 

 notice of a bastard between two sjiecies of this genus. Dytiscidie are rarely found in 

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



C^ 



Fic:. 4(>7. — Dytiscus 

 rcrtlfdlis. 



