52 



MK. K. M. SMITH ON CERTAIN SENSE-ORGANS 



We come now to Eristcdis tenax, the Drone Fly ; this is a very 

 interesting case, possessing as it does two types of sense-pits 

 utterly difterent from each other. One type may be called the 

 Complex Pit and the other the Simple Pit. PI. I. fig. 1 is from a 

 photograph of the former. It illustrates the ver-y abrupt ending 

 of the thick chitin and the very. regular shape and character of 

 the whole sense-organ. Each cell can be seen running up to its 



Text-figure 23. 



Senii-;lia^v; mmatic lono'itudinal section tlivougli tlic auteiuni of Xylota si/lvarmn. 



X 128. 



corresponding sensory process, and the whole organ is isolated by 

 a space i-nnning round the base of the cells and cvittiiig it ofi' from 

 the rest of the tissue. PL I. fig. 2 is from a photograph under 

 a low power of the simple type of pit. It is merelj' a large in- 

 pushing of the chitin widely open to the exterior, and instead of 

 the delicate sensory processes found in other sense-pits, there are 

 large and coarse rods of chitin. Also there is no abrupt thinning 



