358 PHYLUM ARTHROPODA. 
as adults, eg. fleas, lice, bird-lice, plant-lice, etc., or in- 
ternally as larve, eg. 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, — 
Ly 
ra iy Vi Tos 
id ) 
a 
Fic. 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 réle 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 
