INSECTS AND OTHER ORGANISMS 415 



next generation is not due to the germinal constitution of the 

 egg-nucleus, but to a perfectly distinct organism which is 

 able to invade the egg before its maturation. 



Some animal parasites much more highly organised than 

 the protozoa, are found not within but outside the bodies of 

 various insects ; these parasites, though far larger than the 

 Protozoa, are necessarily small when compared with the 

 insects that they infect. A large black dor-beetle (Geo- 

 trupes) is often found with the ventral region of the body 

 and the bases of the hmbs covered with little yellowish 

 mites ; these are young forms of Gamasus coleopterorum, 

 a member of a family of predaceous mites, the larger kinds 

 of which pursue and capture small insects and suck their 

 juices ; these tiny creatures can live in numbers as external 

 parasites of a fairly large beetle. The water-mites 

 (Hydrachnidae) mostly reddish in colour are well known to 

 students of pond-life. The eggs are laid on water-plants 

 and the six-legged larvae are parasitic on various aquatic 

 animals, many species thus making use of the insects that 

 share with them the freshwaters as a dwelling-place. 

 Dragon-flies, aquatic Hemiptera like Nepa, Corixa, and 

 Notonecta, and water-beetles are often found with numbers 

 of hydrachnid larvae attached to various parts of their bodies. 

 C. D. Soar and W. WiUiamson (1925) describe how the 

 ventral surface of a large Dyticus may be '' closely covered 

 with larvae, the first comers attaching themselves to the 

 under side of the head and succeeding attachments being 

 behind these until the ventral surface is covered gradually 

 from before backwards." This parasitism on insects is 

 only temporary during the mites' larval stage ; when adult 

 they feed at the expense of Arthropoda of another class, as 

 they capture the small Crustacea known as " water-fleas." 

 A mite Acarapis apis of the Tarsonemid family has been 

 shown by Rennie (1921) to live and reproduce its kind in 

 the large thoracic air- tubes of the Hive-bee, the mites 

 gaining entrance through the spiracles, pairing and laying 

 their eggs in the tracheal cavities. The walls of the air- 

 tubes become discoloured through the piercing and sucking 



