Mr Thompson, The Cuticula of Insects, ete. 51 
The Cuticula of Insects as a means of defence against 
Parasites. By Wituiam R. THompson. (Communicated by 
Mr F. A. Ports.) 
[Read 23 November 1914.] 
A number of years ago, in his “ Lessons on the Theory of 
Inflammation,” Metchnikotff expressed his belief that the phago- 
eytic reaction to parasitic organisms in Arthropods is in general 
rather feeble; and he suggested that this condition is correlated 
with the development of a chitinous cuticula which covers the 
entire body and lines a great part of the alimentary canal. Since 
this cuticular armour prevents the entrance of parasites, the 
stimulus for the development of the phagocytic function is 
absent. A little later, Cuénot, in his paper on the physiology 
of the Orthoptera, objected to this view, on the ground that the 
cuirass of the Arthropods, in spite of its thickness and its resist- 
ance, has never prevented the penetration of parasites; but that, 
on the contrary, it is just the animals of that group which are 
most heavily infested by internal parasites. The fact that normal 
parasites are not ordinarily molested by the phagocytes—which 
Cuénot had himself observed—he explained not on the basis of a 
feeble phagocytic power in the Arthropoda, but as the result of 
a progressive adaptation of the parasitic organisms which enables 
them to resist or repel the phagocytes by which they would other- 
wise be destroyed. 
The question of the relation between the phagocytes and the 
parasites of the Arthropods is difficult and complex. Such data 
as I have been able to gather up to the present time seem to me 
to support the contention of Metchnikoff. At all events, toward 
the great majority of the internal metazoan parasites of Arthro- 
pods there is no visible direct phagocytic reaction. Without 
treating this matter in detail, I wish now only to consider the 
subsidiary question of the actual value to one group of the 
Arthropoda—the Insects—of the cuticula and its appendages 
as a means of defence against parasitic enemies. 
In this connection the Dipterous and Hymenopterous parasites 
are especially interesting, for they ordinarily gain entrance to the 
body of their hosts through the integument. Those parasites 
which make their way into the haemocoele by the mid-intestine 
are naturally little affected by cuticular developments or func- 
tions. 
The thickness and resistance of the cuticula in the larvae of 
Lepidoptera sometimes effectively prevents the entrance of the 
larvae of Dipterous parasites. I have frequently noticed hatched 
