as & means of defence against Parasites 53 
This parasite penetrates into the body of Huproctis chrysorrhoea 
through an opening which it makes in the skin of the caterpillar. 
Like many common Tachinid parasites it does not withdraw 
completely into the body cavity of the host, but comes to rest 
with its caudal extremity, bearing the respiratory stigmata, in the 
opening through which it has entered. The hypodermal cells in 
this region proliferate, at the same time actively secreting a 
considerable quantity of chitinous material; and there is formed 
a funnel-shaped “integumental sheath” surrounding the body of 
the parasite and opening at its ectal extremity to the exterior. 
I have several times observed that when the ecdysis of the host 
occurs a short time after the penetration of the parasite, the little 
larvae may be withdrawn from the body of the caterpillar in their 
integumental sheaths and cast off with the cuticle to which they 
are attached. Pantel has also found that at a more advanced 
stage, when the parasite larva is much too large to be “moulted 
out” by the host, the integumental sheath may break off at the 
point where it joins the integument, and slip down with the 
parasite which it surrounds into the general body cavity, where 
the Tachinid soon perishes from want of free oxygen. 
As for the Hymenopterous parasites, they possess a well- 
developed apparatus—the ovipositor—by means of which in many 
cases they introduce their eggs directly into the body cavity of 
the host; and their larvae, more perfectly adapted to a parasitic 
existence than the majority of the Dipterous parasites, usually lie 
free in the haemocoele of the host, where they are not affected by 
the process of moulting. However, it is very likely that even 
against these parasites the cuticula is really a very important 
means of defence. 
That the Arthropods support a vast number of internal para- 
sites is undeniable. It may even be true, as Cuénot contended, 
that they suffer more severely in this way than any other group 
of the animal kingdom. In any case, however, it is not therefore 
safe to conclude that this heavy parasitism results from the in- 
adequacy of the cuticula as a means of defence. The efficacy of a 
defensive structure can be determined only in relation to the vigour 
and extent of the attacks which it has to support. It appears 
to me that the question gains in interest if we turn it about and 
ask ourselves whether there is not something specially efficient 
about the parasites of the Arthropoda or some peculiarity of 
structure in the Arthropods themselves which makes them un- 
usually susceptible to parasitic invasion. 
One striking peculiarity of the Arthropods is that the para- 
sitic forms in many cases prey upon other members of their own 
group. To a limited extent, it is true, the same thing can be said 
of other groups of animals. Some of the suctorial infusoria are 
