DIPTEEA. 57 



vated, and terminated by four hairy tentacles ; at the sides of the 

 fifth segment may be observed a little angle, from which projects 

 a horny retractile point. 



It is of very singular habits. It makes a small tunnel in 

 the sand, having a conical mouth, where it waits, like the 

 spider, immovable. As soon as an insect falls into the hole, it 

 raises its head, and squeezing its prey in the folds of its body, 

 devours it, and afterwards throws out the skin. It lives in this 

 way for at least three years before attaining the perfect state. 



The VolucellcR (Fig. 38) have a strong resemblance to 

 the humble bee. Certain kinds make use and abuse of this 

 resemblance to introduce themselves fraudulently into its nests, 

 and to deposit their eggs therein. When these eggs have 

 hatched, the larvae, which have the mouth armed with two man- 



Fig. S8. A species of Volucella. Fig. i>9. A species of Helophilus. 



dibles, devour the larvae of their hosts, the bees. This is the 

 return they make for the hospitality they have received ! 



The Helophili (Fig. 39) deserve to be mentioned here on 

 account of the singular form of many of their larvae. The head 

 is thick, fleshy, and varying a little in form. But the point 

 by which they are easily to be distinguished from most other 

 larvae is that they have always very long tails, sometimes, indeed, 

 out of proportion to the length of the body. Eeaumur called these 

 larvae "vers a queue de rat;" they are known in England as 

 rat-tailed maggots, and their habits are aquatic. Having placed 

 some of them in a basin of water, Eeaumur saw that they kept 

 in a perpendicular position at the bottom of the basin, and parallel 

 to one another, the extremities of their tails being on the sur- 



