918 AN INTRODUCTION TO ENTOMOLOGY 



Lan^ce and pupse of holometabolous insects are the usual hosts. 

 Some braconids parasitize plant lice and a very few attack adult 

 Coleoptera. A few genera of ichneumonids are parasites in the eggs, 

 cocoons or on the bodies of spiders. 



In a typical life history of an ichneumonoid, the adult female 

 locates the host larva by searching in a likely spot, identifies it with 

 a few gentle taps on her antenna, and then quickly deposits an egg 

 inside the larva by inserting her sting-like ovipositor. The larva 

 writhes and thrashes about but is usually not able to drive away the 

 parasite before the deed is done. The parasite's egg soon hatches 

 and the tiny maggot-like larva that issues from it lies in the body 

 cavity of its host. It grows gradually by feeding on the blood and 

 fat body, avoiding the vital organs. The host larva develops in a 

 normal manner but perhaps is unusually lethargic. When the host 

 is nearly ready for pupation the parasite begins more rapid feeding 

 and growth. Soon it begins to gorge itself with the vital organs of 

 its host which rapidly sickens and dies, leaving a limp and shrivelled 

 carcass of which the parasite makes short work. The cocoon is spun 

 by the mature parasite larva either in the remains of its host or a 

 short distance away. Throughout its feeding life the parasite has 

 not excreted any visible wastes. Just before pupation it discharges 

 the accumulated wastes in its intestine in a mass called the meconium. 

 This dries to a hard pellet in the posterior end of the cocoon. Pupa- 

 tion takes place and a few days or weeks later the adult parasite 

 gnaws a hole in one end of its cocoon and escapes fully hardened and 

 colored. This is usually at a time when a new generation of the host 

 is ready for parasitism. 



Some species are obligatory secondary parasites and will feed only 

 on some other parasite. Others are either primary or secondary 

 parasites according to whether or not other parasites are in the hosts 

 in which they develop. 



Instead of living in the inside of their hosts, the larvae of some 

 species are external feeders. In these cases the parasite egg is placed 

 on the host or near it and the larva feeds through cuts in the host's 

 skin. External parasites are liable to be dislodged and therefore are 

 seldom found on exposed and active hosts. The usual victims are 

 larvae or pupae in tunnels, nests, or cocoons. The externally parasitic 

 species usually permanently paralyze the host by a sting from the 

 ovipositor before depositing the egg. This immobilization of the host 

 gives further protection to an externally feeding larv^a. 



An ichneumonoid larva feeding externally upon a paralyzed host 

 in its burrow has habits which differ but little from those of tiphiid 

 wasps whose larvae feed upon scarabseid beetle grubs permanently 

 paralyzed in their burrows by a sting from the parent wasp. It is a 

 short step from this type of life history to that so common among the 

 wasps usually called predators. In these the female provisions a nest 

 lor her offspring with one or several paralyzed insects. The difference 

 is in the amount of care given to her offspring's food by the adult 

 wasp rather than in the habits of the larva itself. It is thus difficult 

 to distinguish between the parasitic and the predacious habit among 

 the Hymenoptera. Recently some authors have been using the term 



