54 Mr Thompson, The Cuticula of Insects 
parasitic in Ciliates: the larvae of several of the Narcomedusae 
(Cunina proboscidea, for example) infest adult Trachymedusae. 
Cooke mentions three or four species of Mollusca said to be para- 
sitic in other Molluscs, while the Hag-fishes and a few Teleosts 
are parasites on fishes. No doubt cases of this sort could be cited | 
in several other phyla, but the fact remains that the vast majority | 
of parasitic animals belonging to groups other than the Arthro-- 
poda—the parasitic Protozoa, Trematodes, Cestodes, Orthonectids, | 
Dicyemids, parasitic Rotifers, Nematodes and Molluses—practi- | 
cally all infest animals of other groups. 
On the other hand, there are among the Crustacea, Arachnida 
and Insects, parasitic groups the members of which do not attack | 
Arthropods—the parasitic Copepods, the Sarcoptids and the - 
Oestrids may be cited. However, among the Crustacea, and | 
especially among the Insects, an equal or much larger number 
of parasites invariably attack more or less closely related species. 
It will suffice in this connection to mention the Rhizocephala, the 
great group of the Epicarid Isopods, the Tachinids, Ichneumonids, 
Braconids and Chalcids. 
Of the manner in which the parasitic habit arose among the 
Arthropods we know nothing. The fact that so many of them prey 
upon one another and that among the Insects true secondary and 
SO Le 
even true tertiary parasitism exists, seems to be one expression of - 
the intense competition among the members of this truly dominant 
group. In any event it seems reasonable to suppose that forms 
which have taken to parasitizing their near relatives have many 
things in their favour. The life cycle of many parasites in other 
groups—such as the Trematodes for example—involves a passage 
from one environment to another, and sometimes includes two or 
more very different hosts. This is at best an uncertain business, 
and even a high rate of reproduction cannot altogether offset the 
chances of misfortune. With but few exceptions, adult Arthropod 
parasites inhabit the same environment as the Arthropod host, 
and comport themselves in it in a somewhat similar manner. No 
Arthrepod parasite passes through more than a single host during 
the course of its life cycle. All of these features contribute to 
make these forms formidable enemies of the other Articulates 
upon which they prey. 
It is also possible that the Arthropods in many cases are 
particularly well adapted to the needs of parasites. For example, 
Wheeler has suggested that the possession by the insects of great 
quantities of reserve material, utilized by the organism during the 
metamorphoses and in the development of the genital organs, and 
the exhaustion of which has but little effect upon the general 
vitality, renders these animals particularly fitted to. support 
parasitic invasions. 
