STRUCTURE, GROWTH AND ECONOMICS OF INSECTS 57 



milking stable after having fed on human excreta is potentially a 

 disease vector, so that we cannot too strongly emphasize the necessity 

 of keeping all foods adequately screened and of preventing such insects 

 as house-flies, blow-flies or mosquitoes from entering human dwellings. 



Besides carrying disease germs many insects may themselves cause 

 disease in man. Many mites, lice and fleas cause dermatitis, scabies 

 or ulcers in man. The larvae of many flies, notably the blue bottles, 

 blow-flies, flesh flies and bot flies, cause intestinal, dermal, muscular, 

 nasal or auricular myiasis in man. 



MacGregor in a recent paper (Bui. Ent. Res., Vol. viii, pp. 155-163) 

 lists eighty organisms causing disease which may be transmitted by 

 insects. 



Insect Behavior Toward Stimuli 



In recent years a large mass of facts regarding the behavior of insects 

 toward their environment — both organic and inorganic — has been 

 collected, and in a few cases this information has been of service in the 

 control of injurious forms. In general, however, the application of such 

 methods of control is still in its infancy stage, but it gives promise of 

 valuable results in the near future. 



As the relations of insects to plants and to other insects have been 

 discussed in previous sections attention will be confined here to the 

 behavior of insects under the influence of environmental stimuli, such as 

 light, heat, moisture, chemical contact, winds, etc. 



For some time it has been known that plants show tropistic move- 

 ments with regard to light, heat, gravity, moisture, contact, etc. 

 Moreover, some progress has been made toward an understanding 

 of the processes. Plants, for example, bend toward the light because 

 the cells on the side away from the light grow faster than those on the 

 side next to the light. There is no conscious control of the movement 

 by the plant. Animals, too, exhibit movements under the influence of 

 tropic or taxic^ stimuh. In the case of insects, butterflies, bees, house- 

 flies, and many moths and caterpillars are positively phototropic or 

 phototactic and move toward the light, while maggots, bed-bugs and 

 cockroaches move away from the light. 



* The term taxic is now more commonly used than Iropic when applied to the 

 locomotor movements of animals under the action of stimuli, Iropic being usually 

 reserved for the turning or orienting movements. 



