DURA MATER. 



[ 217 ] 



DYTISCUS. 



tonemiaceae (Florideous Algae), containing 

 one British species, D. filiformis, 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 wall'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 upon the internal 

 wall of the tube (which is filled up with ge- 

 latinous substance), while the tetraspores 

 are found among the surface-cells. 



BIBL. Harvey, Brit. Alg. p. 147. pi. 20 a ; 

 Phyc. Brit. pi. 59; Greville, Alg. Brit. pi. 17. 



DURA MATER. This strong, somewhat 

 elastic membrane, of a tendinous aspect, 

 consists of white fibrous tissue, with a few 

 nuclear fibres. The former is composed of 

 bundles, interlacing in various directions. 

 The dura mater is lined internally by a layer 

 of flat, polygonal, nucleated, epithelial cells, 

 representing the reflected layer of the arach- 

 noid membrane. 



BIBL. Treatises on Anatomy, Kolliker, 

 Handbuch d. Gewebelehre, and Mikrosko- 

 pische Anatomie. 



DYTISCID.E. A family, and 



DYTISCUS, Linn., or Dyticus.K genus 

 of Coleopterous Insects, belonging to the 

 family Dytiscidae. 



The characters of the family are, antennas 

 long and slender ; external lobe of maxillae 

 articulated; anterior pah- of legs shorter 

 than the posterior pairs, which are flattened 

 and fringed with hairs. Aquatic. 



The genus Dytiscus is characterized by the 

 three first 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 ; 

 one of them, D. marginalis, is common in 

 ponds and pools. 



The head is well adapted for the display of 

 the trophi, or organs of the mouth. The 

 labrum is transverse; the mandibles short 

 and robust, with a strong internal tooth ; 

 the maxillse are short, flat, and ciliated in- 

 ternally, with the tip acute ; the outer lobe 

 palpiform ; the true maxillary palpi are about 

 twice as long as the maxillse ; the mentum is 

 transverse, with the sides produced into two 

 lobes ; the labium is short and square, the 

 palpi about twice its length, and three- 



jointed. The structure of the anterior tarsi 

 in the male (PL 27. fig. 6 a) is very curious ; 

 the three basal joints being expanded late- 

 rally so as to form a broad and rounded patella 

 or shield, convex above, and covered beneath 

 with a number of variously-sized suckers 

 or discs, some of which are stalked (fig. 6 b, 

 one of the smaller ones). This structure 

 enables the male to retain his situation upon 

 the back of the female, the elytra of the lat- 

 ter being furrowed longitudinally, to aid in 

 this effect. The three basal joints of the tarsi 

 of the middle pan* of legs are also flattened 

 beneath, and covered with the stalked disks. 



Fig. 185. 



Dytiscus latus. Natural size. 



The full-grown larvae are about two inches 

 in length. They are of a dark ochre or dirty 

 brown colour ; the body long, subcylindrical, 

 and eleven-jointed ; the two terminal joints 

 are long and conical, the sides of the apex 

 being fringed with hairs. The terminal seg- 

 ment is fiirnished with a pair of long and 

 slender pilose appendages, by means of 

 which the insect can suspend itself at the 

 surface of the water ; these are tubular, and 

 communicate with the tracheae of the body. 

 The head (PI. 28. fig. 14) is large, oval, or 

 rounded, depressed, and with five or six 

 small elevated tubercles near the anterior 

 angles representing the eyes (fig. 14 a). Two 

 rudimentary, slender, seven-jointed antennae 

 (b) are inserted in front of the eyes. The 

 mouth has no aperture; the food, consisting 

 of the juices of the prey, passes through a 

 canal traversing the long, sickle-shaped, 

 acute mandibles (c). The maxillae (d) are 

 slender, cylindrical, and terminated by a 

 short lateral spine ; the maxillary palpi (e) 

 are of the same thickness, arising from the 



