ARTHROPOD A 



Insecta. There are more species of insects known than of all the 

 other groups of animals combined. Their numbers, though variously 

 estimated, run well up in the hundreds of thousands, and the number 

 of undescribed species can hardly be estimated. The study of this 

 enormous class is a large department of science in itself, Entomology, 

 and one of great importance largely because, as a class, insects are 

 highly destructive to vegetation and injurious to man in other ways. 



It has been estimated that the direct annual loss in the United States 

 due to insects is about one billion dollars, besides the losses due to dis- 

 ease-producing and other noxious forms. 



D E 



PIG. 62. Acarina, enlarged. C, follicle mite, Demodex folliculorum. D, 

 itch mite, Sarcoptes scapici. E, sheep-scab mite, Psoroptes communis var. avis. 

 (From Hegner, College Zoology, C and D, after Sedgwick, C, from Mengin, D, from 

 Gudden, E, from Osborn.) 



The relation of insects to agriculture is too large a subject to even 

 touch upon in a book like this; there are whole libraries of entomology 

 dealing with this phase of economic zoology. It may be interesting, 

 however, to briefly discuss a few insects that are harmful to man in 

 other ways, and a few that are highly beneficial. 



Mosquitoes. The part played by mosquitoes in the transmssion 

 to man of malaria, yellow fever, etc., has already been noted in Chapter 

 I. Though to most people all mosquitoes "look alike" there are sev- 

 eral hundred species, and according to Felt (95) there are 50 or more 

 species in New York State. Of these, two or three genera, have al- 

 ready been mentioned; Culex, the common non-malarial forms; Ano- 

 pheles, the malarial mosquito; and Stegomyia, the yellow-fever mosquito. 

 The last named is ordinarily found only in tropical or sub-tropical 



