358 



PHYLUM ARTHROPODA. 



as adults, e.g. fleas, lice, bird-lice, plant-lice, etc., or in- 

 ternally as larvae, e.g. the maggots of bot-flies in sheep, 

 and a great number of borers within plants. 



We need only mention Hessian-fly, phylloxera, Colorado 

 beetle, weevils, locusts, to suggest many more which are of 

 much economic importance as injurious insects. On the 

 other hand, our indebtedness to hive-bee and silk-moth, to 

 cochineal and lac insects, to those which destroy injurious 

 insects, and to those which carry pollen from flower to 

 flower, is obvious. 



Finally, we must at least mention that in ants, bees, 



FIG. 188. Mosquito. After Nuttall and Shipley. 



wasps, and termites we find illustration of various grades of 

 social life, and marvellous exhibitions of instinctive skill as 

 well as some intelligence. 



INSECTS AND DISEASE 



As carriers of disease-germs insects play a very im- 

 portant part. The role of flies as mechanical distributors 

 of anthrax, plague, and other bacterial diseases has been 

 clearly proved. Besides carrying bacilli upon their bodies 

 and leaving them on wounds or food, they also swallow 

 germs, and subsequently deposit them in their excreta in 

 similar situations. Undoubtedly, however, the most serious 

 cases are those of the blood-sucking Diptera which act as 



