38 



INSECTS. 



ant 



pass by those which would succeed in logical order, to 

 study a group of animals that is highly specialized. 



Plan of Structure. A superficial study of the external 

 features of the butterfly will give a general idea of the 

 insect plan of 'structure. Examine a butterfly which has 

 been killed in the cyanide bottle, and observe its horny 

 incasement, an outside skeleton covering the whole sur- 

 face, but thickest and hardest on those parts of the body 



most exposed. This 

 is a very different 

 exterior from that 

 of the sponge or the 

 hydra : such ani- 

 mals, if exposed to 

 the air, would soon 

 die of evaporation, 

 if from no other 

 cause. This hard 

 exterior is an adap- 

 tation to aerial life, 

 as well as a protec- 

 tion against injury. 

 This coat of mail in 

 insects is an out- 

 growth from the skin, and is composed of a horny sub- 

 stance called chitin. It is very light and very tough, 

 two important requisites in the armor of a flying animal. 



Place the butterfly in its erect position, with the head 

 away from you, and compare the right and left sides with 

 reference to the arrangement of parts. Observe that 

 the parts correspond, that the sides are symmetrical. 

 Bilateral symmetry is the term used in zoology to desig- 

 nate such arrangement of parts. What animals have you 

 seen that are not bilaterally symmetrical ? 



trn 



DIAGRAM OF SULPHUR BUTTERFLY (X2), de- 

 nuded of scales, right wings removed: 

 h, head; 1, prothorax; 2, mesothorax; 

 3, metathorax; a, abdomen; w I, left fore 

 wing; w 2, left hind wing; black 's indi- 

 cate the points of attachment of right wings ; 

 ant, antenna ; e, eye ;p, palpus ; m, "tongue ;" 

 ex, coxae of right legs ; tr, trochanter of right 

 hind leg ; /, femur ; t, tibia ; trs, tarsus or 

 foot. 



