DUFOUREA. 



[ 272 ] 



DYTISCUS. 



BiBL. Harvey, Brit. Alg. 154, pi. 21 C ; 

 rhyc. Brit. pis. "l 10, 244; Sachs, Bot. 294.' 



DITOU'REA, Ach.— A genus of Li- 

 cliens. D. mndreporifvrmis occurs in Swit- 

 zerland and Germany. 



DUMOX'TIA, Lamx.— A genus of Crvp- 

 tonemiaceje (Florideous Algse), containing 

 one British species, D.Jiliformis, having a 

 delicate tubular frond, of yellowish.greenish, 

 or purple colour, of variable length and dia- 

 meter, with numerous filiform branches, 

 which are long on short fronds, and short on 

 long fronds ; growing commonly on rocks &c. 

 between tide-marks. The waU of the tube is 

 composed of a double layer of tissue, the 

 outer of roundish cells, the inner of longish 

 cells forming filamentous rows. The spores 

 are attached in clusters to the internal wall 

 of the tube (which is filled up with gela- 

 tinous substance), while the tetraspores are 

 found amcing the surface-cells. 



BiBL. Plarvey, Brit. Alg. 147, pi. 20 a ; 

 Phyc. Brit. pi. o9 ; Greville, Alg. Brit. pi. 

 17. 



DYEING. See Staining. 



D YSPOXTI US, ThoreU.— A genus of Co- 

 pepodous Entomostraca. 



D. sfriatus, in dredgings. (Brady, Copep., 

 Bay Soc. iii. Go.) 



DY^STE'RIA, Huxley.— A genus of 

 Hypotrichous Infusoria, fam. Ervilina. 

 Distinguished by the valves of the carapace 

 being united behind, but below only. Ma- 

 rine ; five species. 



BiBL. Huxlev, Qu. M. Jn. 1857, 78; 

 CI. & Lachm. Inf. 284. 



DYSTERI'NA,C1. & Lachm. = Ervilina. 



DY^TIS'CUS, Linn., or Dg'ticns, a genus 

 of Coleopterous Insects of the family Dy- 

 tiscidpe. 



Characters of the family : antennae long 

 and slender ; external lobe of maxilla; arti- 

 culated ; anterior pair of legs shorter than 

 the posterior pairs, which are flattened and 

 fringed with hairs. Freshwater. 



The genus Dgtiscus is characterized by 

 the first three joints of the anterior tarsi in 

 the male being very large, and expanded 

 into a patella or shield; the didactyle claws; 

 and the maxillary palpi having the second 

 and third joints of equal length. 



The species of Dytiscus are of large size ; 

 D. marginalis is common in ponds and pools. 



The head is well adapted for the display 

 of the trophi, or organs of the moutli. 

 Lii brum transverse ; mandibles short and 

 robust, with a strong internal tooth ; max- 

 illae short, flat, and ciliated internally, with 



the tip acute, the outer lobe palpiform : the 

 true maxillaiy palpi about twice as long as 

 the maxillfe ; mentum transverse, with the 

 sides produced into two lobes ; labium 

 short and square, palpi about twice its 

 length, and three-jointed. The structure 

 of the anterior tarsi in the male is very 

 curious (PI. 34. fig. Go), the three basal 

 joints being expanded laterally so as to 

 form a broad and rounded patella or shield, 

 convex above, and covered beneath with a 

 number of suckers or disks of various sizes, 

 some of which are stalked (fig. 6 6. a small 

 one). This structure enables the male to 

 retain his hold upon the back of the female, 

 the elytra of the latter being furrowed lon- 

 gitudinally, to aid in this effect. The three 

 basal joints of the tarsi of the middle pair 

 of legs are also flattened beneath, and co- 

 vered with the stalked disks. 



Fig. 182. 



Dytiaeus latissimus. Natural size. 



Full-grown larvae about two inches in 

 length ; of a dark ochre or dirty brown co- 

 lour; the body long, subcyliudrical, and 

 eleven-jointed ; the two terminal joints 

 long and conical, the sides of the' apex 

 fringed with hairs. Terminal segment fur- 

 nished with a pair of long and slender pi- 

 lose appendages, by means of which the 

 insect can susjjend itself at the surface of 

 the water ; these are tubular, and commu- 

 nicate with the tracheae of the body. Head 

 (PI. 85. fig. 14) large, oval, or rounded, de- 

 pressed, and with live or six small elevated 

 tubercles near the anterior angles represen- 

 ting the eyes (fig. 14 a). Two rudimentary, 

 slender, seven-jointed antennae (A) are in- 

 serted in front of the eyes. The mouth 



