374 MORPHOLOGY OF THE ABDOMEN IN THE ODONATA 



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Introduction 

 problems suggested by variation in the abdomen 



One of the most marked and constant characters of the Odonata 

 is the elongated form of the abdomen. Nevertheless, there is much 

 variation within the group in the proportions of this body region 

 and in its length relative to that of the thorax. There are, more- 

 over, very great differences in both size and form between the 

 larva and the imago, no species so far as known having the 

 abdomen as long in the former as in the latter. It is usually 

 much wider in the nymph, universally so in the Anisoptera. 

 These facts of abdominal structure are the more striking because 

 they are most unexpected. The rather weak thorax of the larva 

 must certainly be enlarged and perfected to meet the require- 

 ments of so swift, skillful and tireless a 1 ier as the adult dragon- 

 fly: all the changes involved at transformation point to its adapta- 

 tion to light. But no such clear-cut tendencies are evinced in 

 the abdominal changes or reasons manifest for so complete a re- 

 organization. 



The first suggestions of abdominal form as a possil^le adapta- 

 tion to environment came from Dr. Calvert. In a series of papers 

 published between 1910 and 1917 he not only recorded the occur- 

 rence of Odonate larvae living and maturing in the water col- 

 lected between the leaf bases of epiphytic Bromeliads, as several 

 other observers cited by him had done, but he described in detail 

 the growth, moulting, transformation, and habits of Mecisto- 

 gaster modestus Selys, a member of the legion Pseudostigma (sub- 

 family Pseudostigmatinae of Tilh^ard) of the family Agrionidae. 

 In his paper of 1911 he makes this remark regarding the trans- 

 formation of Mecistogaster: ''As will be seen from an examination 

 of them (a series of photographs), the great length of the abdo- 

 men of the imago is a relatively sudden acquisition and is not fore- 

 shadowed by the size of the larva." And again (p. 410) "The 

 excessively long abdomen of the adults of Mecistogaster and its 

 allies (Megaloprepus, Microstigma, Pseudostigma, Anoviisma) 

 may be a special adaptation to the life of their offspring in water 

 containing plants, since the abdomen of the larva of M. m< dcUus 

 is no longer, proportionally, than in other Agrioninae. The 

 space between the leaf of a bromeliad and the leaf next without 

 decreases downward, and if Mecistogastei-'s eggs are deposited 



