DIPTERA. 251 



The peculiar connection of the thorax with the 

 abdomen in the horse-fly makes the latter appear to 

 be sessile, but it is very difl"erent from the true sessile 

 abdomen of the more generalized insects of the first 

 series of orders, and we propose to speak of it as a 

 pseudo-sessile abdomen. The basal portion of the 

 abdominal region has been carried forward and has 

 united with the thorax, thus covering up and reducing 

 to the condition of internal parts the posterior por- 

 tions of the metathorax. That these were originally 

 external is shown by their dark color, and also by the 

 fact that in the more generalized forms of the Dip- 

 tera they are fully exposed. In this way it is possible 

 to conceive of a mode in which an insect with pedun- 

 culated abdomen could have become evolved into one 

 with a pseudo-sessile abdomen, but the difference 

 between this type of sessile abdomen and that of 

 generalized orders, like Orthoptera and Hemiptera, 

 must be clearly borne in mind. It has been produced 

 probably by a process of specialization by reduction 

 out of a pedunculated abdomen, whereas the true 

 sessile abdomen is a primitive Thysanuriform charac- 

 teristic. The abdomen of Tabanus liueola has a light- 

 colored band extending down the middle, as seen in 

 PI. XIL, Fig. 196, by which the species is easily dis- 

 tinguished. Its terminal rings are withdrawn into the 

 body, but can be extended like a telescope and serve 

 as an ovipositor, though a weak one ; as, however, the 

 eggs of most flies are laid in or on soft substances, 

 horny digging implements are not needed. 



The compound eyes (PI. XIL, Fig. 197, ey; Figs. 

 196, 198) make up the greater part of the head : in 



