272 



LECTURES ON BIOLOGY 



FIG. 63. CATERPILLAR COVERED WITH COCOONS OF 



Microgaster glomeratus. 



The females of the Ichneumonidae lay their eggs into the 

 eggs, larvae, pupae, and adults of other insects, but by prefer- 

 ence into caterpillars of different butterflies ; from the eggs 



emerge the grub- 

 like larvae without 

 feet and anus, 

 which forthwith 

 commence to eat 

 up their unwilling 

 host, first devour- 

 ing those parts of 

 the body which 

 are not of vital 

 importance, the 

 large accumulations of fat made by the caterpillar to serve for 

 its own support during the long pupa rest-stage. Owing 

 to this cunning procedure the host remains apparently quite 

 healthy and may even proceed to the pupa stage, but then its 

 fate is sealed, for the larvae, having now devoured the food in 

 the reserve store, mercilessly attack the vital organs. When 

 all the food has been eaten up and the caterpillar is dead, the 

 development of the parasite is usually complete. The method 

 adopted by the ichneumon-fly in depositing her eggs shows a 

 remarkably fine instinct, for large caterpillars receive a consider- 

 able share of eggs, while only a few are deposited into cater- 

 pillars whose small size does mot guarantee a sufficiency of food. 



Year after year the Ichneumonidae 

 take immense toll in the insect world, 

 greatly to the advantage of man, for 

 without their help the insect plague 

 would soon become overwhelming. 



Many wasps place their brood- 

 cells in the soil or decaying wood, 

 but before proceeding to egg-laying 

 they fill the cavity with numerous 

 caterpillars, insects; and spiders, 

 which they paralyze by stinging 

 them (fig. 64). Though alive, these unfortunate animals are 

 unable to escape and are later eaten up literally alive. A 



FIG. 64. Ammophila hirsnta 



PARALYSING A CATERPILLAR. 



