NEMATODA 225 



Other unfavourable conditions. They are small enough, as larvae, to 

 obtain entrance through the stomata of leaves, and sometimes possess 

 dart-like projections of the buccal lining which enable them to pene- 

 trate the cell walls of plants. They feed on cell sap and by their inter- 

 ference with the life of the host plant cause the formation of galls, 

 wilting and withering of the leaves, and stunting of the plant. 



Tylenchus tritici passes through a single generation in the course of 

 the year, and infects wheat. The animal becomes adult when the grain 

 is ripening and a pair, inhabiting a single flower, produce several 

 hundred larvae. Instead of the grain a brown gall is produced, and 

 in this the larvae (after moulting twice) may survive for at least 

 twenty years. If the grain falls to the ground the larvae may remain 

 there over the winter or may escape into the soil. When the corn 

 begins to grow in the spring they enter the tissues of the plant and 

 make their way up the stem to the flower, where they speedily mature. 

 The great interest of this life history lies in the easy adaptation of the 

 parasitic life history to the annual cycle of the wheat plant and the 

 extreme capacity for survival in a dormant and desiccated condition 

 until the right plant host becomes available. Tylenchus devastatriXy 

 on the other hand, may pass through several generations in the year 

 and attacks indiscriminately clover, narcissi bulbs and onions, and 

 many other useful plants. Heterodera (Fig. 171 D) is a parasite of the 

 roots of tomatoes, cucumbers and beets, and is remarkable because 

 the female attaches herself in larval life to a rootlet from which she 

 sucks a continuous flow of sap. She is fertilized by wandering males 

 and grows enormously, becoming lemon-shaped. Inside the body 

 thousands of larvae are produced, which escape into the soil and live 

 there until the opportunity arises for infection of fresh roots. 



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

 histories are also of great interest. 



In Mermis (Fig. 169) 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 and wander into the body cavity where they nourish them- 

 selves 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 produc- 

 tion 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 



BI 15 



