THE PROBOSCIS IN THE MUSCIDAE 37 



is further altered as a result of the protrusion of a part of the wall of 

 the head capsule, containing the pharynx, to form a part of the pro- 

 boscis, using the term in the wider sense. 



As the question of the origin of the blood-sucking Muscids has arisen 

 in connection with the phylogeny of the parasites which they and their 

 non-biting allies may harbour it will be well to refer briefly to it here. 

 It will also render the somewhat complex structure and mechanism of 

 the parts more easy to explain, if they are looked at from the compara- 

 tive point of view. 



The relations between the mouth parts of the Orthorraphic flies and 

 those of simpler insects, such as the cockroach, have already been pointed 



out, and the argument may be taken up at the point 



,,.,,, T .u i u Origin of the Blood- 



reached with Tabamts. In this insect not only are the 8uck j n g n/| U8C jdae 



mandibles and first maxillae present and functional, 

 but the labium has assumed a function not found in other flies of 

 the same group. The inner surfaces of the labella are provided with a 

 complex arrangement of grooves, by means of which the fly can and does 

 suck up moisture from surfaces, exactly as does Musca. The form may 

 be regarded, in fact, as having arrived at the parting of the ways, so far 

 as its method of feeding is concerned. With such a condition of 

 the mouth parts two courses of evolution suggest themselves. Either 

 the fly may remain a blood-sucker, obtaining its food from a wound 

 made by the mandibles and first maxillae, retaining the pseudotracheal 

 membrane as an accessory structure, or perhaps losing it altogether; 

 or it may come more and more to use its oral lobes, and to depend 

 on them for obtaining the whole of its food, so that in course of time 

 the piercing parts disappear. This is what appears to have occurred in 

 the production of the Muscid type of proboscis, now found in a very 

 large number of genera. 



The blood-sucking Muscids present no points in common with the 

 blood-sucking Orthorrapha, but are evidently closely related to the 

 non-blood-sucking Muscidae, as shown by their other characters. 

 It is clear from the study of the minute structure of the proboscis 

 that the assumption of the blood-sucking habit has occurred subse- 

 quently to the loss of the mandibles and first maxillae. The 

 essential cutting weapons are new structures, having their origin in 

 the inner walls of the labella, as minute chitinous processes. In 

 Musca domestica and its allies they appear to be sufficiently strong to 

 act as scraping teeth, capable of scratching a surface from which a 

 fluid food will exude ; in one or two other species known they are 



