PROBOSCIS IN THE BLOOD-SUCKING MUSCIDAE 47 



with the blood of the insect, forced down from the haustellum and the 

 rostrum by respiratory movements, aided probably by the transverse 

 muscle at the lower end of the labium. The discal sclerite is rotated 

 by the anterior set of muscles until the prestomum which it surrounds 

 is parallel with the food surface. The furca is pulled upwards by the 

 posterior muscles, and, as the external walls are pulled out of the way, 

 the internal walls are pulled apart and into a plane perpendicular to that 

 which they occupy in the resting position, until they are parallel with the 

 food surface. When in this position the oral surface of the labella forms 

 a pad, which, by virtue of its fluid contents, is easily applied to even an 

 irregular surface. The fluid can then be sucked through the interbifid 

 grooves and up the food canal, by the muscles of the pharynx. 



That the oral lobes are really distended with blood can be easily demon- 

 strated by laying an etherized fly on a slide, and compressing the head 

 gently, without using sufficient force to rupture the contents. First, 

 owing to the pressure driving the air out of the air sacs in the 

 head into those in the rostrum, the latter is extended. Then, on 

 continuing the pressure, the oral lobes are distended, and the pseudo- 

 tracheal membrane exposed. Sometimes a small trachea ruptures in 

 the manreuvre, and a minute air bubble can be seen to float to the 

 uppermost surface. If the pseudotracheal membrane is punctured with 

 a fine needle while it is distended a drop of a rather viscid yellowish 

 fluid exudes. This is the blood of the fly. 



The key to the structure and mechanism of the proboscis in the 

 blood-sucking Muscidae lies in the recognition of the homology of the 

 parts with those of the non-biting forms, such as Musca. 



The cutting organs by which the wound is made are The Blood - 8uckin * 



1-11 11 i r r Muscidae 



the teeth, which are highly elaborated forms of the 



prestomal teeth as seen in Musca. 



Before going into the details of structure it will be of advantage 

 to discuss for a moment the directions in which change is to be looked 

 for in the adaptation of the Muscid proboscis to the blood-sucking habit. 

 The first essential is a cutting apparatus, and it is found, therefore, that 

 the teeth become increased in size and strength even in the earliest 

 stages of the evolution. This necessitates increased strength in the 

 structures which support them, and in the muscles and joints through 

 which they come into play. Both the anterior and posterior joints there- 

 fore become consolidated, and the discal sclerite undergoes early and 

 marked changes. In the second place, a long and narrow proboscis, ter- 

 minating in a point, is essential if it is actually to pierce the skin, and 



