500 MODERN CLASSIFICATION OF INSECTS. 
racter, regarding them as abdominal appendages, in accordance with 
their theory relative to the hind part of the thorax of the Diptera and 
Hymenoptera; and Messrs. Kirby and Spence consider them as ap- 
pendages of the respiratory organs. From a careful investigation of 
their structure and position in the chief types of the order, as well as 
from a comparison of the Dipterous and Hymenopterous thorax, it 
appears to me that the same arguments by which I have endeavoured, 
in a previous page, to prove that the hind part of the middle portion 
of the body in the petiolated Hymenoptera is thoracic, are equally 
applicable to these insects, in order to show, not only that such is also 
the case in the Diptera, but also that the halteres are the real ana- 
logues of the hind wings; and, consequently, that the alulets are 
merely appendages of, or, in fact, not distinct organs from, the upper 
wings. Latreille, indeed, in several of his latest works, instanced the 
larger Tipulidae as showing that the halteres cannot be analogous to 
the hinder wings, inasmuch as they are attached to the segment, 
which bears a pair of spiracles, which, he asserts (but erroneously ), is 
never the case with the metathorax ; but, on carefully examining Ti- 
pula oleracea, it is quite impossible to arrive at any other conclusion 
than that the segment which bears the halteres and pair of spiracles 
is also that which bears the hind legs, and which is consequently the 
metathorax. Moreover, it unfortunately happens, for the theory of 
the French entomologists, that the abdomen of this and other allied 
insects possesses the full complement of segments, without taking this 
supposed basal segment into the calculation. I have represented the 
various developments of these thoracic organs in several Dipterous 
insects in the following figures. 
The legs are long, and terminated by a 5-jointed tarsus, having two 
claws at the extremity, with two or three fleshy vesicles or pulvilli.* 
The abdomen is united to the thorax by a considerable portion of its 
* These pulvilli, or, as Derham terms them, “skinny palms to the feet,” 
have been generally regarded as the instruments wherewith, by means of the pres- 
sure of the atmosphere, flies are enabled to creep on the upright surfaces of glass 
and other polished surfaces against gravity. Mr. Blackwall has, however, lately 
published a very ingenious paper, proving that it is impossible, from the structure 
of these organs, covered as they are with minute bristles, to be employed as 
suckers, and suggesting that it is by strictly mechanical means, as suggested by 
Dr. Hooke (Micrographia, p.171.), that they are enabled to retain their hold, 
(Linn. Trans. vol. xvi. p. 490.) In a subsequent communication, he however con- 
siders that an adhesive secretion is emitted by the hair-like appendage on the inferior 
surface of the pulvilli, which are considered to be tubular; a distinct track of this 
secretion being discoverable in every instance. (Linn. Trans. vol. xvi. p. 768.) 
