DIPTERA. 57 
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 Volucelle (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 eges have hatched, 
the larve, which have the mouth armed with two mandibles, 


Fig. 38.—A species of Volucella. Fig. 39.—A species of Helophilus. 
devour the larvz 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 larve. 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 
larvee is, that they have always very long tails, sometimes, indeed, 
out of proportion to the length of the body. Réaumur called these 
larvee “vers 4 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, Réaumur 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 surface 
