DIPTERA. 



65 



press tents into a wound they wish to enlarge. Two or three days 

 before the worm wishes to come out, it commences to make use of 

 its posterior part as a tent, to increase the size of its exit from its 

 habitation. It thrusts it into the hole and draws it out again many 

 times in the course of two or three days, and the oftener this is re- 

 peated, the longer it is able to retain its posterior end in the opening, 

 as the hole becomes larger. On the day preceding that on which 

 the worm is to come out, the posterior part is to be found almost con- 

 tinually in the hole. At last, it comes out backwards, and falls to 



I the ground, when it gets under a stone, or buries itself in the turf; 

 remaining quiet and preparing for its last transformation. Its skin 

 hardens, the rings disappear, and it becomes black. Thenceforth the 

 insect is detached from the outer skin, which forms a cocoon, or box. 

 At the front and upper part of the cocoon is a triangular piece, which 

 the fly gets rid of when it is in a fit state to come into the open air." 

 Fig. 47, taken from drawings in Reaumur's Memoirs, represents the 

 imago of the Gistrus leaving 

 the cocoon. 



The reader is, most likely, 

 desirous to know with the aid 

 of what instrument the CEstriis 

 is able to pierce the thick skin 

 of the ox. 



The female only is pos- 

 sessed of this instrument, which 

 is situated in the posterior ex- 

 tremity of the body. It is of 

 a shiny blackish brown colour, 

 and as it were covered with 

 scales. By pressing the abdo- 

 men of the fly between one's 

 two fingers it is thrust out. 

 Re'aumur observed that it was 



; formed of four tubes, which 



:: could be drawn the one into 

 the other, like the tubes of a telescope (Fig. 48). The last of these 

 appears to terminate in five small scaly knobs, which are not placed 



: on the same line, but are the ends of five different parts. Three 

 of these knobs are furnished with points, which form an instrument 

 well fitted to operate upon a hard thick skin. United together, they 

 form a cavity similar to that of an auger, and terminating in the form 



Tof a spoon. 

 i 60 



- Fig. 47. 

 Imago of Bot-fly 

 emerging. 



Fig. 48. 



Ovipositor of the Bot-fly 



{CEstrus bovis). 



