AND HABITS OF THE YELLOW DUNG-FLY. 



641 



are to be contrasted with the spines covering the remainder of 

 the bod}'-, which also help to give the segments a grip on the sides 

 of the lai-val burrow, but, being fixed, prevent a backward move- 

 ment. If the anterior segments wei-e covered with these, the 



Text-figure 9. 



Larva of S. stercoraria. 3i-d instar. 

 Text-figure 10. 



Posterior spiracles. 3rd instar. 



larva, once having pushed its head in one direction, would be 

 obliged to continue whether it wnnted to or not. The hinged 

 plates give it a chance to make a second investigation in another 

 direction provided that it has not gone too far. 



