STRUCTURE, GROWTH AND ECONOMICS OF INSECTS 47 



mutual dependence, and that there are grades of fitness on both sides" 

 (Needham, General Biology). Reference has already been made in the 

 section dealing with Beneflcial Insects to the important part played 

 by many insects in the fertilization of plants. To this end many 

 beautiful adaptations occur among plants such as in legumes, iris, milk- 

 weed, yucca, orchids, mints, figworts, honeysuckles, canna, Smyrna 

 fig, etc., but it should be borne in mind that there has been also 'probably 

 much adaptation on the part of the insects. 



Galls. — Another type of inter-relation is the galls seen on many 

 plants, produced by certain insects belonging to the families Cecido- 

 myiidee, Trypetidce, Aphididce, Psyllidce, Cynipida and Tenthredinidce. 

 Mites (Acarina) also produce galls. Usually an egg is laid within the 

 growing tissue and the larva excites the surrounding tissue to abnormal 

 growth. The transformations occur within the gall, and the adult 

 escapes to make new galls. 



Galls are of various forms, often characteristic of the insects pro- 

 ducing them. The nutritive cells lying next to the contained larva 

 contain both sugar and starch and appear to function as feeders for both 

 the larva and the growing cells of the gall. 



Insectivorous Plants. — Certain plants such as the sundew, Venus' 

 fly-trap, pitcher-plant and bladderwort entrap small insects and feed 

 upon them. These plants secrete digestive fluids which convert the 

 tissues of the captured insects into liquid food capable of being 

 absorbed. 



Diseases of Insects. — Many insects are killed by the action of cer- 

 tain fungi and bacteria. Such diseases are frequently epidemic, and 

 attempts have been made to control insect infestations by the propaga- 

 tion and distribution of artificial cultures. Probably the best known 

 experiment of this nature was carried out by Dr. Snow and Dr. Forbes 

 against the chinch bug in the middle States by the distribution of 

 cultures of the fungus S porotrichum globuliferum. The result was only 

 moderately successful, for it was found that the disease spreads rapidly 

 in moist seasons, but not in dry ones when the chinch bugs are inju- 

 rious. There is no doubt, however, that the fungus is an important 

 factor in lessening the severity of the insect's attacks. 



In South Africa, Algeria and Argentina the locust has been con- 

 trolled by the introduction of bacterial cultures of Coccohacillus 



