THE BLOW-FLY. o/ 



ment. These are shown in Figs. 13 and 14. It is not until the 

 larva has somewhat grown that the anterior spiracles are at all 

 developed. Both the spiracle itself, and the tracheal branch that 

 connects it with the main trunk are totally wanting in the newly- 

 hatched larva. The development of this tracheal branch is 

 denoted by the appearance of its epithelial coating, as yet devoid 

 of any cavity or spiral fibre. 



Of the muscles I have little to say, beyond the fact that the 

 internal surface of the integument is clothed with a layer of mus- 

 cular bands, arranged chiefly in a longitudinal direction, and 

 consisting of striated muscular fibres enclosed in muscle-sheaths 

 (myolemma). They are attached at either end to the integument, 

 and by them the movements of the body are effected. Tracheal 

 twigs can be seen distributed over the surface of the myolemma, 

 but I do not think they penetrate it. Nerve-filaments are also 

 doubtless distributed to the muscles, though I have not as yet 

 traced them in this insect. 



The pulsations of the dorsal vessel can be seen through the in- 

 tegument on the dorsal surface. It is largest towards the posterior end 

 of the body. I may mention that in the corresponding part of 

 the dorsal vessel of a Chironomus larva I have distinctly seen 

 valvular slits opening and closing with each pulsation ; probably, 

 the same thing occurs in this larva. Colourless blood-corpuscles 

 may be seen in motion in those parts of the body-cavity which are 

 sufficiently transparent to admit of it. 



The nervous system presents an exceptional and very con- 

 densed form. Instead of a chain of ganglia connected by a 

 double nervous cord, as in Tanypis, and indeed in most other 

 insects, the ganglia are all collected together to form one large 

 nervous mass, surrounding the oesophagus and situated in the 

 third segment (Fig. 15). The two large spherical ganglia, g.g., in 

 this figure represent the cephalic ganglia or brain of other insects, 

 here, however, removed from the first segment, so that the larva 

 can scarcely be said to have a head. This is probably connected 

 with the almost entire suppression of special sensory organs, which 

 are represented by two pairs of papillae on the front of the head 

 (see Fig. 16), beneath which are spherical masses of ganglion cells 

 connected by large trunks to the nerve-centres. The remainder of 



