1916] 



Forbes — On the Tympanum of Certain Lepidoptera 



183 



of the worker is longer and has a reflected occipital margin which is 

 lacking in pubiventris and angusta. 



The observations of INIr. Gaste and Dr. von Ihering throw light 

 on one of the methods of ant dispersal. Of course, the estab- 

 lishment of a species of ant on an island does not require the 

 conveyance of a whole colony as in this instance, since a single 

 fecundated female enclosed in a vegetable cavity might, if trans- 

 ported in a similar manner, suffice for the establishment of a 

 species. 



ON THE TYMPANUM OF CERTAIN LEPIDOPTERA. 



By William T. M. Forbes, 

 Ithaca, N. Y. 



At the base of the abdomen in many Lepidoptera there is a 

 curious and complex organ, which has been referred to but rarely in 

 the literature, and which is known, in the Geometrida?, at least, 

 as the tympanum. So far as I have discovered, its function is 

 unknown, but its location and general 

 structure justify the guess that it is 

 the resonator for an auditory organ, 

 corresponding to the well-known one 

 in the Hemiptera. It should prove an 

 interesting problem for the histologist 

 and physiologist. The present article 

 will not consider its function, but will 

 outline its structure in the principal 

 families of Macrolepidoptera, and offer 

 a couple of suggestions as to their 

 relationship. 



A brief examination shows that there 

 are several analogous, but not homol- 

 ogous organs of this type in the vari- 

 ous families, indicating parallel lines of 



descent from a form that presumably , ^'^- 1- Side view of meta- 



, J . , thorax and base ot abdomen of 



had neither, but may have possessed a Apatelodes forrefacta (Euptero- 



wholly internal sense-organ,— which tidae),showingpriinitivemacro- 



, . , 1 • P lepidopteroiis arrangement of 



served as a stnnulus to their forma- sclerities. 



