B52 - Mr Thompson, The Cuticula of Insects 
eggs of Tachinid flies upon the bodies of insects which proved on 
dissection to be quite free from parasites. In the woods about | 
Boston, Massachusetts, one often finds caterpillars of the Gipsy | 
Moth (Porthetria dispar L.) bearing considerable numbers of the | 
eggs of a native Tachinid parasite (possibly Tachina mella W.). | 
Of a large number of such caterpillars which I once reared, | 
less than one-half per cent. produced parasite larvae. In some © 
cases I have observed—as C. H. T. Townsend has described— | 
the young larvae of these Tachinids, just hatched, struggling | 
vigorously to bore into the body of the Gipsy larva, but | 
apparently quite unable to cut through the thick and heavily | 
ehitinized cuticula. . | 
When the ovoviviparous Tachinid Carcelia cheloniae Rond | 
attempts to oviposit upon the skin of its host, the caterpillar of | 
the Brown-tail Moth (Huproctis chrysorrhoea L.), the thin-shelled | 
eggs are rather often pierced by the fine bristles with which the | 
nettling hairs of the host are beset, while the larvae which emerge | 
from the uninjured eggs are sometimes impaled upon the barbs | 
of the hairs while they are attempting to descend to the skin. 
Though these observations were made in the laboratory, I do | 
not doubt that the same fate often overtakes the parasites in 
natural surroundings. 
Upon several occasions I have collected Datanid caterpillars — 
of which the head capsule and the dorsal shield of the last seg- — 
ment were almost covered by a mass of the large white eggs of 
some Tachinid parasite which I do not think I have ever reared. 
It is evident that larvae hatching from eggs in these positions 
very rarely succeed in penetrating into the body of the host. 
The explanation of this curious fact is very simple. Whenever 
the Datanid caterpillars are disturbed, they elevate the anterior 
and posterior segments so that the body is curved in a semi-circle, 
and in this position they remain for some time rigid and 
motionless. The result of this movement is the presentation 
of the anterior surface of the head and the supra-anal shield to — 
any Tachinid which happens to be hovering above, and the para- 
site is thus misled into depositing its eggs upon the most resistant — 
and impenetrable parts of the caterpillar’s anatomy. | 
It is well known that by the process of moulting, which is in | 
the most strict relation with the development of a chitinous 
cuticula, the larvae and nymphs of. insects often succeed in 
liberating themselves from the unhatched eggs of Dipterous 
parasites. 
Even when the parasite larvae have made an entrance into its 
body they may still be affected by the casting of their host's 
cuticula. In this way the caterpillars of the Brown-tail Moth 
sometimes rid themselves of the larvae of Carcelia chelonae. 
