CANKERWOr.MS SNODGRASS 



327 



noted there that the reduction of (he jaws, which organs are large 

 and strong in the caterpiUar and rudimentary in the moth, takes 

 place by two steps, the first in the cliange from the caterpillar to 

 the pupa, the second in that from the pupa to the moth. The wings 

 of the female cankerworm moth undergo likewise their final reduc- 

 tion during the change from the pupa to the moth, but since the 

 caterpillar is wingless, the wings appear first on the pupa, where 

 they form fairly large pads as in the pupae of moths that are to 

 liave fully developed wings. We might ask why wings are formed 

 at all when they are only to be reduced again. The answer is that 

 the maternal ancestors of the modern cankerworms were fully winged, 

 and that the pupa in acquiring wings is but repeating the evolution 

 of the species. 



The tendency to \ / \^\ y^/ 



wing reduction in the 

 female moths of the 

 Geometridffi, like the 

 tendency to leg reduc- 

 tion in the caterpil- 

 lars, appears to be a 

 physiological procliv- 

 ity of the family; it 

 crops out in various 

 degrees of expression 

 in different species, 

 though in some spe- 

 cies both males and 

 females are fully 

 winged. The female of a species called the half-winged geometer 

 (fig. 13, B) has wings of about half the normal size, appendages 

 probably useless, but showing that the moth has reached a halfway 

 stage in the evolution toward winglessness. In another species, 

 known as Bruce's measuring worm, the female (fig. 13, A) resem- 

 bles the females of the cankerworms but has wing rudiments a little 

 longer. It is difficult to imagine just what such innate tendencies 

 within a group of animals may be in terms of physiology, yet many 

 other examples of the same thing might be given among insects 

 and other animals, just as we note among ourselves family traits 

 and family tendencies with regard to physical as well as tempera- 

 mental characters. But again, wing reduction in moths, like leg re- 

 duction in caterpillars, is not limited to the Geometridse, the female 

 of the common tussock moth being as nearly wingless as the female 

 canker moths. 



Summer passes, fall comes, the first of November arrives. The 

 place is southern Connecticut. The days are again cold and bleak. 



Fig. 13. — Female Geometrid moths with wings in differ- 

 ent degrees of reduction. A, moth of Bruce's meas- 

 uring worm {Bachela bruceata) having wings a little 

 larger tlian the cankerworm moths ; B, moth of the 

 half-winged geometer with wings neai'ly half the nor- 

 mal length (X 21/2) 



