PARASITIC INSECTS. 
15 
Fig. 2.— Head of a Noctuid cater- 
pillar on the hickory, containing a 
freshly-hatched ichneumon larva. 
A, d, egg-shell of the ichueumon 
on the caterpillar's head, the larva 
(e) having bored into the piotho- 
racic segment of its host. B as 
the host appears ten minutes 
later, the egg-shell bavins dropped 
off. The prothoracic segment has 
contracted and the bead has be- 
come swollen, while the posterior 
part of the caterpillar's bead has 
concealed the opening of the lar- 
val parasite seen at A, e. Gissler, 
del. 
Fig. 2 represents the mode of oviposition 
by au unknown ichneumon observed by us 
in Providence. The egg (d) was laid on the 
head, and the larva soou hatching, bored 
under the skin, entering the body so as 
finally to disappear out of sight. 
The eggs are laid either within or on the 
outside of the body of the host, usually 
some caterpillar. 
A special account of the mode of egg-lay - 
iug of au European ichneumon (Paniscus 
cephalotes) is given by Mr. E. B. Poulton in 
the Transactions of the Entomological So- 
ciety of London, 1886, page 162. It laid 14 
eggs on the caterpillar it selected as its 
host, firmly attaching them to its skin, most 
of them in the sutures between the segments 
on the sides of the body. 
"It is probable that an excess of ova is generally laid, for a small 
proportion do not develop, and the way in which they are attached in 
small groups insures that of those that do develop a large proportion 
of the larvae are so crowded by the others that they die at an early 
stage, as has been also previously observed. If too large a number 
were laid and all developed, it is obvious that none could arrive at ma- 
turity; but this is obviated iu the manner described above, and it is 
partly brought about by the limited space on the circumference of the 
larva attacked. This space, of course, varies with the size of the lat- 
ter, and it is more quickly filled in the rapid development of the para- 
sites upon small than upon large larvae; so that, if they are too numer- 
ous, crowdiug ensues earlier, and with more fatal results in the former 
than in the latter case. Thus the smaller surface may compensate for 
the less amount of food, and may itself insure that the parasites reach 
maturity." The ichneumon lays a smaller number of eggs on small 
caterpillars than on large ones, and yet lays more than can develop in 
all cases, "the eggs beiug laid in such a way that crowding results if 
all or nearly all develop; so that the chance of the eggs being sterile 
is obviated on the one hand and of the parasitic larvae dying immature 
on the other." 
The larva of the ichneumon does not attack the solid or vital parts 
of its host, but absorbs the blood and other fluids of the body. Mr. 
Poulton thinks that the motive force which drives the blood from the 
body of the host into the digestive tract of the parasite is entirely 
supplied by the contracted body- walls of the former. 
Many ichneumons are polyphagous, i. e., live in insects of widely differ- 
ent species, and those of different orders.* Others confine their attacks 
* This and the following remarks on ichneumons are taken mainly from Judeich 
and Nitsche's Lehrhuch der Mittel-Enropiiischen Forstiusektenkunde. 
