32 



trunks before entering the throat. The proboscis is strengthened across the middle by a 

 strong muscular band, which probably serves the purpose of dilating and contracting it. 



The next point of interest is the antennae. These are small horns situated beyond the 

 base of the proboscis, and are covered with hairs. It has long been and still is a disputed 

 point among naturalists, as to what functions these organs perform. Some attribute to 

 them the sense of hearing, others of smell, and others again that of touch. In the case 

 of moths it has been shown by actual experiment that these organs serve to direct the 

 flight. Dr. Clemens, an American entomologist, has experimented on the cercopia moth, 

 and he found that the excision of one antenna made the flight of the insect very irregular 

 and unnatural, while the loss of both seemed to have deprived it of almost all power in 

 guiding itself in the air. Whether these organs serve the same purpose in the case of 

 Musca domestica we are uncertain, but it is probable they do. 



The eye will next occupy our attention, and it is an object well worthy of it. To the 

 unaided vision it may present no extraordinary appearance, but when examined skilfully 

 by the aid of a microscope, its true structure is revealed in all its beauty. The house fly 

 has two eyes which are situated one on each side of the head ; they are, comparatively 

 speaking, of enormous size, as they occupy a very large portion of the head. Each eye 

 is compound, consisting of about two thousand separate lenses, each one of which is com- 

 plete in itself, and capable of conveying a distinct impression to the sensory organs. Un- 

 til recently it was a question much discussed among naturalists, whether these remarkable 

 compound eyes of the fly conveyed to the nerve centres of that insect one or many ima- 

 ges of objects presented to them. Many v/ere of opinion that each lens conveyed a dis- 

 tinct image, but of late it has been shown that such is not necessarily the case, for although 

 it can be clearly demonstrated that each facet receives a distinct image, there can be little 

 doubt but that the various images meet at a common centre, and are conveyed to the sen- 

 sory organs as a single picture. Our own eyes aff'ord us an illustration of this principle, 

 for even if we look at one object with both eyes, and a distinct image is reflected on the 

 retina of each, yet we do not see two objects, but only one distinct image. Therefore the 

 immense number of eyes the fly has, while giving it an enormous range of vision, need 

 not necessarily yield a confused impression. 



We will next consider the breathing apparatus of Musca domestica. This consists of 

 a series of air tubes or tracheae in the body, terminating externally in trap doors, or spi- 

 racles as they are technically termed. The tracheae are small tubes which branch in all di- 

 rections, and are found distributed freely throughout the body. The air from without en- 

 ters freely through the trap doors above mentioned, and traverses these tubes, thereby 

 performing exactly the same function as the liings of man. Under the microscope these 

 tracheae reveal a most wonderful structure, which we will refer to in the language of Pro- 

 fessor Rymer Jones : — 



" There is one elegant arrangement connected with the breathing tubes of an insect 

 especially worthy of admiration ; and perhaps in the whole range of animal mechanics it 

 would be difficult to point out an example of more exquisite mechanism, whether we con- 

 sider the object of the contrivance, or the remarkable beauty of the structure employed. 

 The air tubes themselves are necessarily extremely thin and delicate, so that on the slightest 

 pressure their sides would inevitably collapse, and thus completely put a stop to the passage 

 of air through them, producing of course speedy sufi'ocation of the insect had not some 

 means been adopted to keep them always^ permeable ; and yet to do so, and at the same 

 time to preserve their softness and perfect flexibility, might seem a problem not easily solved. 

 The plan adopted, however, fully combines both these requisites. Between the two thin 

 layers of membrane, which form the walls of every air tube, a delicate elastic thread (a wire 

 of exquisite tenuity) has been interposed, which winding round and round in close spirals, 

 forms by its revolutions a cylindrical pipe of sufficient firmness to preserve the air vessels 

 in a permeable condition, whilst at the same time it does not at all interfere with its flex- 

 ibility ; this fine coil is continued through every division of the tracheae^ even to their 

 most minute ramifications, a character whereby these vessels are readily distinguish- 

 able when examined under the microscope." 



Man has imitated this exquisite contrivance in the spiral wire spring which lines 

 flexible gas-pipes ; but his wire does not pass between the two coats of membrane. One 

 of the most interesting points of the contrivance is the way in which the branches are (so 



