PARASITISM 347 



its eggs in the tomato worm and other Sphin- 

 gidae. The eggs hatch, the larvae live upon the 

 fat and tissues until mature, when they bore 

 through the skin and spin tiny cocoons which 

 the caterpillar carries fastened to its back, giving 

 its body the appearance of being covered with 

 rice grains. 



Many species are provided with remarkably long 

 ovipositors with which to reach and lay their eggs 

 in larvae, which may lie hidden away under bark, 

 in burrows in wood, or in tunnels in the earth, or 

 with which they bore through cocoons to reach 

 the insect hosts at the beginning of the stage of 

 pupation. 



Some of these insects (Megarhyssa lunator and 

 Pimpla) are large and robust, measuring 18 to 20 

 mm., and are provided with ovipositors, 10 cm. or 

 more in length; others, the Proctotrypidae, are so 

 tiny as to be enumerated among the smallest 

 known insects. Such minute insects oviposit in 

 the eggs of other insects and of spiders. 

 The hymenopterous parasites are of immense 

 benefit to man by holding in check his chief 

 insect enemies, especially coleopterous and lepi- 

 dopterous insects. For the destruction of the 

 cotton worm and the Hessian fly we are entirely 

 under obligation to them. 



It is interesting to observe in conclusion that 

 the parasitic habit is so largely developed among 

 the hymenoptera that hyper-parasites i.e., para- 

 sites of parasites may reach even the third and 

 fourth degree. 



Class Arachnida. Of this class which in- 

 cludes the ticks and mites, the scorpions and 

 spiders, we find practically all of the ticks and 

 one-half of the mites to be parasitic, either 

 upon animals or upon plants. 

 The ticks comprise two families, the Argasidae 

 and the Ixodidse which form the superfamily 



