TYPE TB ACHE AT A. 501 



Finally, in some of the two-winged flies the pupa is enclosed 

 within the last larval skin, possessing then a cylindrical form 

 without any indication of the adult limbs {pupa coarctata). 

 A metamorphosis in which a distinct pupa-stage occurs is said 

 to be "complete" in contradistinction to the hemimetabolic 

 form frequently spoken of as " incomplete." 



Mention should be made here ot the dimorphism or polymorphism which 

 occurs in certain adult Insects. It has already received passing mention 

 (p. 497), but in addition to the frequently-occurring sexual dimorphism 

 there occurs in forms which live together in colonies a polymorphism asso- 

 ciated with a division of labor on the parts of the members of the colony. 

 Thus in the Bees there are found the drones or males with heavy bodies, 

 the queen or female, as large as the drones but with a much more slender 

 body, and the workers, which are sterile females distinguished by their 

 smaller size and by other features, such as a peculiar modification of the 

 tibias of the last pair of legs which adapt them for the collection of pollen 

 from the flowers which they visit. Among the Ants a similar trimorphism 

 occurs, males, females, and neuters or workers constituting the colony; and 

 in some tropical forms the workers are of two kinds, namely, ordinary 

 workers with small heads and mandibles, and soldiers with large heads and 

 strong prominent mandibles, whose functions are indicated by the popular 

 name applied to them, though guards would perhaps be more appropriate. 

 Finally, among the Termites, popularly known as the White Ants, four 

 forms, i.e., males, females, workers, and soldiers, also occur. 



In certain Butterflies a peculiar form of dimorphism or trimorphism 

 termed "seasonal dimorphism" occurs, an excellent example of it being 

 offered by the American Papilio AJax, of which there have been described 

 three distinct varieties, differing markedly in coloration both in the males 

 and the females, and distinguished as the varieties Walshii, Telamonides 

 and Marcellus. From chrysalids which have passed the winter there hatch 

 out in the early days of spring forms belonging to the variety Walshii, and 

 somewhat later, from those whose development has been retarded, the 

 Telamonides forms. During the early part of summer the Walshii forms 

 die out and a little later the Telamonides also disappear, both forms pre- 

 viously, however, depositing ova, most of which develop into larvae and 

 chrysalids and hatch out in the later months of summer as the ITarcellus 

 form, whose ova, again developing into chrysalids, pass the winter in that 

 state, and give rise in the following spring successively to the Walshii and 

 Telamonides forms. The three varieties are evidently produced by influ- 

 ences acting upon the chrysalis and differing according to the season, per- 

 haps according to temperature, whence the distinguishing name applied to 

 this form of dimorphism, which is also said to occur in certain Spiders. 



