434 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1931 



parasitic insects feeds upon its host, which ordinarily lives only as 

 long as necessary for the parasite to become mature. The parasites 

 do not feed u^jon the vital organs of the host, but instead are nour- 

 ished from the blood which flows about through the open spaces 

 of the host's body, and from the fat body which represents the re- 

 serve food supply of the victim. While residing in the host's body 

 the hymenopterous parasite larva retains in its alimentary canal, 

 which is saclike and closed behind, all the wastes from the digestive 

 process. The waste materials are voided concurrently with the last 

 moulting. 



It is a significant fact that all parasitic insects attacking others of 

 their class have complete metamorphosis. The larva is no doubt 

 more adaptable to a multitude of circumstances of life than the 

 nymph tj'pe of young, such as is present in grasshopper life cycles. 

 While limited in locomotor capacity, the larva has a flexible body 

 capable of entering the soil or boring into and out of a host, and by 

 its tenacious hold on the host, or by virtue of its position in the host, 

 ceases to use, and has long since lost, all the legs it ever had. The 

 parasitic larvae have been engaged in the business of living at the 

 expense of others a long time, as witness the reduction of the legs, 

 antennae, and mouth parts. That they once possessed these organs 

 is suggested by the facts that some nonparasitic relatives still have 

 them and that some parasitic larvae, notably of the ichneumonoid 

 flies, retain their large falcate mandibles in the first larval instar but 

 lose them when they moult the first time. While the nymphs of such 

 external parasites as the sucking lice of mammals and the chewing 

 lice of birds are parasitic, they do not exhibit the versatility in choice 

 of and fitness for life in a considerable variety of host situations such 

 as hymenopterous and dipterous parasitic larvae display. Among 

 the many thousands of species of parasitic insects there is material 

 for a very interesting study of the multiplicity of form and habit 

 changes or adaptations, such as the means by which the parent gets 

 its progeny upon or into the host, how and in what stage the parasite 

 emerges again, and the variety of hosts it may attack. In a single 

 superfamily, the ichneumonoids, we find one species an ectoparasite 

 on caterpillars, another an internal parasite in a hard-shelled, swift- 

 running beetle, a third in a minute, sluggish, soft-bodied plant louse, 

 and a fourth more than a hundred times larger than the former and 

 carrying a set of drills much longer than itself for reaching into the 

 burrow of a tree borer which is the larva of another member of its 

 own order. In fact, one stage or more of some member or members 

 of practically all the 24 orders of insects are probably subject to 

 attack by one species or another of parasitic insects. Furthermore, 

 some parasitic larvae have come to attack other parasite larvae, and 



