TEIDACTYWS. 203 



in the form of a small segment, which is parted in the middle * (though not always 

 projecting beyond the true segment). This appendage is certainly homologous with 

 the inferior valvae of the ovipositor, which are formed by the connecting membrane of 

 the eighth segment. The last ventral segment is parted by a ridge, a character 

 indicating a tendency of this segment to divide into two parts, so as to form the 

 superior valvse of the ovipositor. The structure of the eighth and ninth (seventh 

 and eighth as seen without dissection) segments shows a rudimentary, but evident, 

 tendency to the formation of an ovipositor f. In the males (Tab. XI. fig. 2) the 

 last two ventral segments are simple, without any such modification, and the terminal 

 one is almost square. 



Nevertheless, the two sexes are not always easy to distinguish on account of the 

 characters of the females being rather variable. Generally, the appendix of the eighth 

 segment is not produced and apparent, and in some species the notch of this segment 

 is obsolete — e. g. in T. histrionicus, the eighth segment in this insect being, on the 

 contrary, produced in the middle, instead of notched (Tab. XI. fig. 1). 



The differences between the sexes are best summarized as follows: ? , last ventral 

 segment usually parted by a ridge, the hind margin of the preceding segment notched 

 or produced in the middle ; <s , the last ventral segment entire, the penultimate 

 segment transverse, entire. — In both sexes the cerci (Tab. XL figg. 1, 2, 3, c) are two- 

 jointed, and the anal appendages (Tab. XI. figg. 1, 2, 3, a) are entire and cylindrical. 



Several species of Tridaetylus have been characterized only by their markings and 

 by the length of the wings ; but the colour is of slight importance in this genus, the 

 yellow markings being subject to disappear altogether, and the wings are very variable 

 in their length, tending to atrophy in specimens of the same species. The real 

 distinctive characters are to be found in the size, in the armature of the hind tibiae, 

 and in the length of the posterior metatarsus ; also, in some species, in the form of the 

 anterior tibiae ( c? ), or in the structure of the last ventral segments ( 2 ) J. 



The Tridactyli are found abundantly, and with similar forms, in all parts of the 

 world, except apparently Australia. 



* Comp. Saussure & Zehntner, Revue Suisse de Zoologie, ii. p. 411, t. 16. fig. 20 u. 



f Comp. Saussure & Zehntner, I. c. p. 411. 



J It has been admitted that in some Tridactyli the anterior and intermediate tarsi are composed of three, 

 and in others of two joints only. A careful examination of all the species in my possession has proved that 

 the tarsi do not in any case possess more than two joints. The illusion of their possessing three joints is easy 

 to explain by the fact that the first joint of the intermediate tarsi is lobed (Tab. XI. fig. 5), and appears under 

 the magnifying-glass as if divided into two parts, and also by the fact that the lobulum in which it terminates 

 is often accidentally curved downwards and takes the appearance of an additional joint. Examination 

 under the microscope explains these appearances. In consequence of this, the classification of the genus, 

 as established formerly by Burmeister and by myself, should be modified : the divisions Xya and Tridaetylus, 

 Sauss., will form a single division only, and the name Xya, as the more recent, must be excluded. The 

 Central-American species must thus be co-ordinated with the synopsis here given. 



2 Dd2 



