PARASITIC INSECTS. 



15 



la 



Fig. 2.— Head of a Moctuid cater- 

 pillar on the hickory, containing a 

 freshly-hatched ichneumon larva. 

 4, d, ejig-3liell of the ichneumon 

 on the caterpillar's head, the larva 

 (e) having bored into the piotho- 

 racic segment of its host. B, aa 

 the host appears ten iiiinntes 

 later, tlieegg-shell havina dropped 

 ofl'. The prothoracic segment has 

 contracted and the head has he- 

 come swollen, while the posterior 

 part of the caterpillar's head has 

 concealed the opening of the lar- 

 val parasite seen at A, e. Gissler, 

 del. 



Fig. 2 represents the mode of ovipositiou 

 by au unknown ichneumon observed by us 

 in Providence. The egg {d) was laid on the 

 head, and the larva soon 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- 

 ing of an 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 in 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, crowding 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 being 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 Nitscbe's Lehrbuch der Mittel-Europiiiscben Forstinsekteukunde. 



