76 Searle, Gleanings of a City Naturalist. [voi.'^xxxvi. 



is grated off by the teeth and dissolved by a salival fluid which 

 the fly pours upon it from its saHvary ducts, and then the 

 sweet solution is sucked back again. When the food is first 

 swallowed it passes into a crop or sucking stomach. But it 

 does not remain there very long. It is soon brought back 

 again and is swallowed once more. This time it goes down 

 the alimentary tube proper, and flows on till it arrives at the 

 spherical-shaped proventriculus, which has sometimes been 

 described as a gizzard. The proventriculus is capable of being 

 closed during the early part of a meal, in order that the food 

 may not enter the intestine, but pass into the crop. It also 

 opens when it is necessary to allow material to pass into the 

 intestines. These observations, made by Graham-Smith, seem 

 to indicate that the proventriculus acts as a valve, and not, 

 as stated by Lowne, " a gizzard and nothing more." The long 

 vessel called the ventriculus is the true digesting stomach. 

 This tapers off into a coiled intestine, and ending in the rectum, 

 or receptacle for waste material. 



In the second of the chief divisions — the thorax — we find 

 ourselves able to gain a better conception of the shape of these 

 smaller segments, joints, or rings which are the final sub- 

 divisions of the bodies of insects. If the middle part of the 

 thorax be examined there will plainly be seen all the parts of 

 which an insect segment can consist. On the upper surface 

 is the dorsal plate, at the sides two lateral plates, and under- 

 neath the ventral plate. The openings at the top show where 

 the wings are attached, while beneath are the attachments 

 of one pair of legs. The wings are made up of a double 

 membrane, and are, in fact, a kind of flattened bag or sac, 

 which is strengthened at places by folds called veins or nervures, 

 and the areas between are called cells. The main veins run 

 longitudinally from base to the top of the wing, but there are 

 some cross veins. The differences in the arrangement of the 

 veins afford ready means of distinguishing M. domestic a from 

 other flies often found in houses. On the hind margin of 

 the wing, near the base, there is a more or less free lobe called 

 the alula. Internal to the posterior lobule of the wing are 

 placed smaller membranous plates known as squama and 

 antisquama. The squama is thicker than the rest of the wing, 

 and is attached posteriorly to the wing-root. Possibly these 

 facilitate the opening and closing of the wings. Behind 

 the wings the pair of halteres — commonly called balancers or 

 poisers — is placed, the most characteristic of all dipterous 

 structures. Tliey are l)elievcd to be the homologues of the 

 hind pair of wings, though their exact .functions are far from 

 clear. Each consists of a conical base provided with a number 

 of sense organs ; on this base is mounted a slender rod ; at the 



