128 B. T. LOWNE ON THE PROBOSCIS OF THE FLY. 



outer chitinous structure developed upon it. These are permanently 

 separated, in such insects as Tabanus, in which the outer layer 

 forms a sheath for the lancets, and the inner alone enters into the 

 structure of the proboscis. In the blow-fly, both layers are united 

 in the adult insect, and give attachment to the muscles which move 

 the lips ; they terminate in long processes twisted at their junction 

 with the maxillse in such a manner as to form springs, which keep 

 the lips of the proboscis closed when at rest ; a powerful muscle is 

 inserted into them, which overcomes their elasticity, and opens the 

 lips, adding another to the already numerous known instances in 

 which elasticity is opposed to muscular force, where prolonged 

 muscular action would otherwise be necessary. 



Perhaps I may be excused if I here mention two or three well- 

 known but important facts. Insects are supplied with a circulation 

 of air just as vertebrates are with a circulation of fluids, and 

 although I do not deny that the fluids of insects circulate, their 

 circulation is exceedingly imperfect and very sluggish. Now the 

 tracheal tubes of insects are by no means permanent in every case, 

 rapidly growing structures are supplied with an immense number 

 of trachese, which disappear a few hours after their appearance, 

 when the structures they supply are fully developed. Secreting 

 and excreting organs are permanently supplied with a large number 

 of tracheal tubes, but the number varies according to the activity of 

 the organ. 



When the fly first emerges from the pupa, none of the muscles 

 of the proboscis are developed, nor are any of the chitinous struc- 

 tures hardened or opaque; the proboscis is filled with a milky cor- 

 puscular fluid, from which these parts are developed, it is then 

 largely supplied with trachese, and hangs down, as the insect is 

 quite incapable of moving it. In an hour or two the muscles are 

 developed and the insect retracts the organ, soon after its wings 

 are sufficiently dry to enable it to fly away. Most of the trachea? of 

 the proboscis disappear as soon as the chitinous parts are hardened. 



Tracing back the oesophagus it will be found to bifurcate after 

 entering the thorax, one division passing upward and back- 

 ward into a muscular proventriculus and so into the stomach, the 

 other passing backward along the under surface of that viscus 

 enters the abdomen, near the base of which it opens into two large 

 abdominal crops, which, when filled with fluid, occupy nearly half 

 that cavity. 



