138 MANUAL OF ANIMAL BIOLOGY 



spread typhoid, tuberculosis, and other serious diseases in this 

 manner ; or in many cases the Insect may carry the germs of the 

 disease internally and transmit the latter by injecting the germs 

 directly into the body of the victim as, for example, in a certain 

 species of the Mosquito which transmits Malaria in this manner. 

 (W. pp. 339-342.) 



On the other hand, it should be noted that there are a number 

 of species of Insects, such, for example, as the Honey Bee, the 

 Lac Insect, and the Silk Worm, which contribute very materially 

 to man's welfare. Commercially the most important is the Silk 

 Worm, and it is from the cocoon of this animal, spun at the end 

 of the larval period, that the silk of commerce is obtained. This 

 is done by unwinding the silky thread which makes up the cocoon, 

 after having first killed the pupa by placing in boiling water. It is 

 stated that the length of thread that may be unwound from an 

 average cocoon is 1526 feet, or somewhat more than one-fourth of a 

 mile. The larvae of many other Insects spin a cocoon, but the 

 ' silk ' is without value either because it is not of good quality or 

 because it cannot be unwound. (W. f. 267.) 



TEXTBOOK BEFEBENCES 



Woodruff, pp. 91-97 ; 325-335 ; 403-417. 



Curtis and Guthrie, pp. 342-396. 

 Guyer, pp. 219-249 ; 683-689. 

 Hegner, pp. 282-352. 

 Newman, pp. 252-269 ; 387-390. 

 Shull, pp. 245 ; 252 ; 256. 



GENEBAL BEFEBENCES 



Lurz. Fieldbook of Insects (Putmans). 



Metcalf and Flint. Fundamentals of Insect Life (McGraw-Hill). 



Parker and Ha swell. Textbook of Zoology (Macmillan). 



Sharp. " Insects," in the Cambridge Natural History (Macmillan). 



Snodgrass. Anatomy and Physiology of the Honeybee (McGraw-Hill). 



Wheeler. Ants (Columbia Univ. Press). 



