226 



HUBERT M. MORRIS. 



pair being slightly nearer the anterior end than the remainder. On its ventral 

 surface this segment bears two rows of processes, the first row containing 

 six and the second four. The segments five to ten are similar to the fourth 

 segment. 



On the dorsal surface of the eleventh segment there is a row of six large pro- 

 cesses near its posterior margin, of which the median pair is the largest. There is 

 a single lateral process on each side near the anterior border of the segment and on 

 the ventral side there are six processes. 



The twelfth segment bears a pair of large spiracles anteriorly, which are placed 

 in a more dorsal position than those on the other segments. Posteriorly it bears 

 a row of four large dorsal processes, the median pair being the largest 

 on the body. On the ventral surface of this segment there is a single pair of 

 processes. 





O 

 c 



4 



o 





m 





Fig. 10. Portion of cuticle of Bibio marci larva from dorsal 



surface of fourth segment ; a, lateral view ; b. surface view ; 



the arrows directed anteriorly ; x 330. 



The cuticle with which the body is covered bears many small scale-like structures, 

 which are packed especially closely together on the processes. The largest of these 

 scales bear stout backwardly-directed spines of a darker colour than the remainder 

 of the cuticle (fig. 10). The scales on the first, second and third segments, and 

 some on the ventral surface of the twelfth segment, bear from one to four spines, 

 and those on the remaining segments usually carry a single stout spine, but a second 

 more slender spine is present on certain of the scales. The scales on the base of the 

 processes all carry several spines, but those on the upper part have only a single 

 spine. 



The alimentary canal (fig. 11) takes an almost straight course through the 

 body, but has a loop near the posterior end, in the hind gut. The largest part of 

 the alimentary canal is the mesenteron, which bears three large caeca at its anterior 

 end, one lying on each side and the third, which is the largest, placed ventrally. 



