210 



OUR HOUSEHOLD INSECTS 



to the black bristles which are scattered over the body 

 at large. The abdomen is very pretty, but is difficult to 

 describe, inasmuch as the distribution of colour varies 

 with the position in which it is seen. On each segment 

 there is a pattern of patches with zig-zag outlines, some 

 of which appear ashy grey, and the rest deep brownish- 

 black ; but it depends entirely upon the light which of 

 them shall appear grey and which brown. The pale 

 tint is that of the tomentum, while the dark is the real 

 colour of the skin, and according to the angle at which 

 the insect is placed, the one or the other becomes more 

 plainly visible. Thus, if the fly be turned towards the 

 source of light, the patches marked a 2 and b 1 in Fig. 68 



appear brown, and those 

 marked a v b 2 pale grey ; but 

 if it be turned round in the 

 opposite direction, the colours 

 are reversed, and a 2 ^ appear 

 pale and a x b 2 dark. 



Very little is known of the 

 earlier stages of this fly, but 

 the perfect insect is often 

 common, and occasionally 

 even becomes a great annoy- 

 ance. The worst instance 

 on record is one from the 

 United States, where the insect is called the "Cluster 

 Fly." Professor Riley gives the following extract from 

 a letter descriptive of the visitation which occurred at 

 Geneva, N.Y. : " They were at once a terror to all neat 

 housekeepers, and from their peculiar habits a constant 

 surprise. People soon learned to look for them every- 

 where in beds, in pillow-slips, under table-covers, 

 behind pictures, in wardrobes, nestled in bonnets and 



FIG. 68. Abdomen of Pollenia 

 rudis, female. 



