118 GENERAL ZOOLOGY 



The chalcid-flies (Chalcididce) do great good to man by 

 laying their eggs in insects, particularly caterpillars. 

 The eggs hatch in or on the host and the larvae burrow about 

 among the internal organs. Sometimes in the summer one 

 may find a dead caterpillar covered with little white 

 cocoons. These contain the pupae of chalcids which 

 lived within the caterpillar as larvae. 



The ichneumon-flies (Ichneumonidce) are also parasitic 

 on caterpillars and other insects. There are many spe- 

 cies and most of them attack only one host. Thalessa 

 lunator (Fig. 57) is an interesting parasite on the larvae 

 of the pigeon horn-tail, Tremex columba. These larvae 

 are found in the heart wood of trees. The female Thalessa 

 bores into a tree with her slender ovipositor and lays her 

 eggs in them. 



There are several families of wasps. The most primi- 

 tive species are diggers, carpenters, or masons. Some 

 tunnel in the ground, others make galleries in wood, or 

 plaster pellets of mud together to form nests. Such 

 cavities are stored with spiders, caterpillars, and other 

 insects. An egg is deposited with accumulated food and 

 each cavity is sealed up. The sting of a wasp contains a 

 poison which paralyzes the nervous system of its prey. 

 The insects placed in the cells with the eggs are not killed, 

 but only paralyzed. They thus remain in "cold storage" 

 until the wasp's larva is ready to devour them. The social 

 wasps (Vespidce) live in colonies. They do not seal up the 

 egg with stored food, but care for and feed the larva soon 

 after it hatches. The greater part of each colony dies 

 when winter comes. Single fertile females hibernate in 

 rotten stumps, in barns, under logs, and in similar situa- 

 tions. In spring each builds a little nest and begins to lay 

 eggs. As soon as the young wasps emerge from the pupal 

 condition they begin to help enlarge the nest, gather food, 

 and care for the young. The original queen, the males, 

 and the "workers" (which are sexually immature females) 

 all die in the autumn. Before the colony breaks up, how- 



