56 



OUR SHADE TREES AND THEIR INSECT DEFOLIATORS. 



ber of them. But the fly now to be described is fully as useful as any 

 of the others. 



"Taehina-fiies are very easily overlooked, because they resemble 

 large house-flies, both in appearance aud in flight, and their presence 

 out of doors is not usually noticed on that account. Yet they play a 

 very important role, living as they do in their larval state entirely 

 in insects. During the caterpillar plague such flies were often seen 

 to dart repeatedly at an intended victim, buzz about it, and quickly 

 disappear. If the caterpillar thus attacked was investigated, from 

 one to four yellowish-white, ovoid, polished, and tough eggs would be 

 found, usually fastened upon its neck, or some spot where they could 

 not readily be reached. These eggs are glued so tightly to the skin of 

 the caterpillar that they can not easily be removed. Sometimes as many 

 as seven eggs could be counted upon a single caterpillar, showing a 

 faulty instinct of the fly or flies, because the victim is not large enough 

 to furnish food for so many voracious maggots. If the victim happens 

 to be near a molt, it casts its skin with the eggs and escapes a slow but 

 sure death. But usually the eggs hatch so soon that the small maggots 

 have time to enter the body of the caterpillar, where they soon reach 

 their full growth, after which they force their way through the skin 

 and drop to the ground, into which they enter to shrink into a brown, 

 tun-like object (known technically as the coarctate pupa), which con- 

 tains the true pupa. The caterpillar, tormented by enemies feeding 

 within it, stops feeding and wanders about for a long time until it dies. 

 As a rule not more than two maggots of this fly mature in tbeir host, and 

 generally but one. The caterpillar attacked by a Tachina-fly is always 

 either fully grown or nearly so. 



"Tachina- flies abounded during the whole term of the prevalence of 

 the caterpillars. But it is impossible to state positively whether they 



were all bred from them or not, since the 

 many species of this genus of flies resemble 

 each other so closely that a very scrutiniz- 

 ing investigation would have been necessary 

 to settle such a question. But there is no 

 doubt that they were very numerous during 

 the summer. Some maggots obtained from 

 caterpillars kept for this purpose in breeding 

 jars changed to the fly in six days: others 

 appeared in twenty-three days, aud still 

 others, obtained at about the same time, are 

 still under ground, where they will hibernate. The maggots of these 

 flies do not, however, always enter the ground, as some were found in- 

 side cocoons made by caterpillars among rubbish above ground." 



We have found, moreover, that three of these primary parasites of 

 the Web-worm, viz, the Apanteles, the Limneria, and the Meteor us, were 

 killed off at a serious rate late in the season by secondary parasites, 



Fig. 26.— A Tacliina-fly. 



