256 THE INVERTEBRATA 



Insect parasites. Four of these may be mentioned, though other life 

 histories are also of great interest. 



In Mermis (Fig. 176) a curious reversal of the typical nematode 

 life cycle occurs. The sexual forms are all free-living either in the soil 

 or fresh water. On summer days after showers the sexual forms of 

 Mermis nigrescens exhibit a curious tropism, leaving their haunts two 

 or three feet in the ground and crawling up the stems of plants, but 

 disappearing when the sun grows warm. The eggs are laid in the 

 ground and when the larvae hatch they pierce the skin of insect larvae 



Fig. 178. Insect and plant parasites. A, Atractonema. Female showing the 

 beginning of the prolapsus of the uterus, which has proceeded in Spherularta, 

 B and C, until it is far larger than the rest of the worm. In C, the body is a 

 minute appendage (bd.) of the prolapsed uterus (ut.pr.) not much longer than 

 one of the greatly enlarged cells of the latter. D, Heterodera ; i and 2, larvae 

 attached to a rootlet with their heads imbedded in its tissues ; 3, the full-grown 

 female (on a smaller scale), removed from the plant. The alimentary canal is 

 shown in black to emphasize that its growth c auses the increase in size of the 

 parasite. 0. ovary; Mi. uterus. A-C, after Leuckart; D, after Strubell. 



and wander into the body cavity where they nourish themselves by 

 absorbing fluid food through the cuticle. The mid gut has become a 

 solid body, having no connection with the mouth and anus, and in it 

 fat is stored up which serves as raw material for the production of 

 eggs. When the animals become sexually mature they escape into the 

 soil. 



In Tylenchus dispar (a form which is thus placed in the same genus 

 as the well-known plant parasites) the adult female and innumerable 

 larvae are found in the body cavity of the bark beetle, Ips, during the 

 winter. Allantonema has similar relations to another bark beetle, 

 Hylobius. The female is enormously developed, the uterus and other 



