DIGESTIVE SYSTEM OF PERIODICAL CICADA. 215 



interesting to note that this food reservoir is similar in most 

 respects to the enlargement in the digestive tube of the cicada, 

 and furthermore that it attains its enormous dimensions only in 

 the later adult life of the insect. In this last respect it further 

 resembles the condition in the cicada. 



From the above accounts taken with other works not quoted 

 here, it seems evident that the digestive systems of the related 

 forms of the group under consideration studied to date have, in 

 general, the same ground plan and that their specializations 

 although varying in degree, are mostly of the same kind. These 

 structural complications have led to different interpretations of 

 the relationships and functions of the various organs, but it 

 seems likely that there is a much closer similarity in these respects 

 than at first appeared to be the case. 



The conclusions here presented are based upon a study of 

 nymphs of Tibicen septendecim Linn, of four different sizes, 

 viz. 2^2 mm.; 7 im.; 14 mm.; and 21-24 mm. This series covers 

 practically the entire period during which the insect lives under 

 ground. The 2^2 mm. nymphs are only recently hatched from 

 the egg, while the 21-24 mm. individuals were full grown nymphs 

 dug from the ground about one month previous to the general 

 emergence and transformation of the brood. According to 

 Marlatt, the nymphs of 7 mm. body length are about four years 

 old, while the 14 mm. ones are about ten years of age. These 

 relationships between size and age were established by workers 

 in the Bureau of Entomology, who followed a complete life- 

 cycle of one of the cicada broods. 



The method of serial sectioning and reconstruction was followed 

 throughout this study. The impossibility of tracing the alimen- 

 tary canal by means of gross dissections is even more apparent 

 in the immature forms than it is the adult. 



In describing the digestive organs as found in cicada nymphs 

 of different ages, it would be more logical to begin with the 

 youngest and follow the developemental series until the adult 

 condition is reached. However, in an earlier paper the adult 

 digestive organs were described and the descriptions and com- 

 parisons here given have been written keeping the adult structure 

 in mind. Obviously, in such a situation it will be easier for the 

 writer and more understandable for the reader to work back- 



