THE INSECT WORLD. 



349 



are wrinkled and hairy when examined with a magnifying glass, 

 soft in texture, thickened behind, and tapering almost to a point 

 at the head, which is marked only by a pair of hooks and a 

 little circular opening representing the mouth. They have no 

 legs, and move by extending the body forward as far as pos- 

 sible, then clinging with the anterior segments to the leaf or 

 twig, and drawing the balance to meet the head. Awkwardly 

 as they move, however, their progress is yet sufficiently rapid 

 for their purpose. When once a larva has established itself in 

 a colony of plant-lice, it never stirs until all in its immediate 

 vicinity are destroyed ; it then moves only far enough to bring 

 into reach additional prey, and so continues until no more re- 

 main. The female lays her eggs close to, or actually among, 

 an aphid colony, so that the larva finds food ready at hand 

 as soon as it is hatched. It grasps a plant-louse with the 

 mouth parts, lifts it from the surface, and sucks 

 its juices, leaving the creature to struggle for 

 a time, helplessly kicking its legs in mid-air. 

 When the juices are exhausted the empty shell 

 is dropped and another specimen is taken. 

 When full-grown, the larva draws itself up 

 into a humped mass ; the outer skin hardens, Syiphus larva de- 

 darkens in color, and forms an apparently solid yo^rmg a piant- 



111-11 



covermg or coarctate pupa, beneath which the 



true or soft pupa of the fly is formed. Several broods of these 

 predaceous flies occur in the course of the season, and they are 

 among the most important checks that nature has provided 

 against plant-lice increase. 



Others of the species are not quite so useful, and occasionally 

 we have a form that is almost parasitic, living in nests of bum- 

 ble-bees or other Hymenopterous insects and feeding upon 

 their larvae. In such cases we often find that the flies greatly 

 resemble in appearance the hosts among which they live. 

 Among the feeders in vegetable matter we have a great variety 

 of form, but they are usually more or less maggot-like and 

 without legs. As they are not of particular economic interest, 

 no more attention need be paid them here. The larva of the 

 "drone-" or "chrysanthemum-fly," Eristalis tefiax, lives in 

 masses of soft, decaying, or excrementitious matter. It is fre- 



