108 



FOURTH ANNUAL REPORT OF 



than the males, for I have bred no less than seven of the former and nut 

 a single one of the latter sex. The female, according to Mr. Trouvelot, 

 deposits from eight to ten eggs upon the skin of her victim, and the young 

 larvte soon batch from them and commence to prey upon the fatty parts 

 of the worm. Eiit as only one of the. parasitic larva^ can tind food suifi- 

 l^^'"'^' ■'' 1 cient to mature, the rest all die from hun- 



ger, or else are devoured b}' the strong- 

 est one which survives them. At first 

 one would suppose that this deposition 

 of several eggs by the parent Ichneumon^ 

 where only a single larva can develop, is 

 a striking instance of misdirected instinct j 

 but we find a similar prodigality through- 

 out Xature, for every individual is so sub- 

 ject to disasters of one kind or another 

 in its struggle for existence that a pro- 

 vision of several ova is often necessary to 

 insure the future d evelopment of a sin- 

 gle one, just as we often sow several seeds 

 of some particular ])lant, in order to insure the growth of a single one. 



x\fter the Cecropia worm has formed its 

 cocoon, the parasitic larva, which had 

 hitherto fed on the fatty jjortions of its 

 ^ victim, now attacks the vital parts, and, 

 when nothing but the empty, skin of the 

 worm is left, spins its own cocoon, which is oblong-oval, dark brown inclin- 

 ing to bronze, and spun so closel}" and compactl}', that the inner layers when 

 sejDarated have the appearance of gold-beater's skin. If we cut open one 

 of these cocoons soon after it is completed, we shall find inside a large fat 

 legless grub (Fig. 38), which sometimes undergoes its transformations and 

 issues as a fly in the fall, but more generally waits till the following spring. 

 The Cecropia Tachina-fly — (Exorista lexicanice, Kirk. var. cecrojjice. 

 Eiley) — The Ichneumon-fly last mentioned usually causes a dwarfed appear- 

 ance of the wonn which it infests, and parasitized cocoons can generally be 

 distinguished from healthy ones by their smaller size. The laryse of this 

 Tachina-fly, wdiich is also parasitic on the Cecropia worm, seem to produce 

 an exactly opposite effect — namely, an undue and unnatural growth of their 

 victim. In the beginning of September, 1866, I received from Eockford 

 Ills., an enormous Cecropia worm. It measured over four inches, was a full 

 inch in diameter, and weighed nearly two ounces ; but like many other large 

 specimens wdiieh I have seen since, it was covered with small oval opaque 

 white egg-shells, clusters of four or five occurring on the back of each seg- 

 m.ent, invariabl}^ deposited in a transverse direction. The skin of the worm 

 Avas black where the young parasites had hatched and penetrated. This 

 large worm soon died and rotted, and in about twelve da^'s a host of mag- 

 gots gnawed their way through the putrid skin. These maggots averaged 



