COMPARATIVE ANATOMY. 



330 



loped. The fore wings are tho moat constant and generally the 



largest, hut in on.- family (Stn-p.iiptera) they are rudimentary, 

 mill in another lariri- unL-r (Coleoptera) they are converted into 

 hard covers for tho hind wings, and never employed in flight. 

 Tho liiml wings, though largely developed in the beetle and 

 grasshopper, and quite as large as the fore wings in dragon -lli.--i, 

 n only adjuncts to the fore wings, being much smaller 

 than these, and fastened to them or dissevered from them as the 

 insect wills. Thoro are various and elaborate contrivances by 

 which this j unction of the hind wings to the fore ones is effected in 

 insects. In flies, tho hind wings are reduced to little sticks with 

 knobs at the end, and why they should bo retained at all in thin 

 - :i puzzle to entomologists. The abdomen or hind division 

 of the body is elongated, and tapering towards tho end. It con- 

 sists of iiiiio segments, but the last two or three or four are often 

 reduced in size, and applied, not to contain the viscera, but to 

 purposes of reproduction, defence, etc. Thus, in the bee, the 

 sting consists of two modified rings, and the ovipositor of tho 

 saw-fly is of the same nature. The rings of tho abdomen are 

 not firmly applied to one another along their edges, as is the 

 case with the rings of the thorax, but tho front one overlaps the 

 hind ones, and these can be retracted one into the other like tho 

 joints of a telescope. The muscles running from one ring to 

 another, which retract the hinder joints into those before them, 

 are so elastic, and they originate so far forward into the front 

 segment that tho whole abdomen may often be at one time 

 doubly as long as at another time. This looseness in the joint- 

 ing ia not only found to be very useful to the insect in 

 order to enable it to bend that part of the body, and so apply 

 the tail organs as the insect requires, but also it allows the in- 

 ternal organs to be distended without inconvenience; and in 

 many of the most active insects a rhythmical breathing is 

 observed, caused by the shortening and elongating of this part 

 of the body. 



Tho food-canal of insects ia usually not very long or very 

 complex. From the mouth a narrow throat runs right through 

 to the abdomen, thus interfering as little as possible with the 

 play of the muscles of that part. When it has arrived at that 

 division of the body, it enlarges into a bulb or crop, which is 

 sometimes, as in bees, an enlargement lying in the track of the 

 canal, and sometimes a bag or reservoir connected with tho 

 throat by a narrow duct which enters sideways into the canal. 

 Below this, or occupying its place when it is absent, a gizzard is 

 often found, whose horny internal longitudinal ridges grind the 

 food. Below this is the true stomach a long sack with trans- 

 verse wrinkles, in the folds of which secreting glands are found. 

 At the lower end of this, a number of long tubes enter. Six of 

 these are seen in the beetle in the illustration, and in the bee 

 there are a large number. These long tubes, which lie in tho 

 body cavity tangled and twisted among the other organs, com- 

 mence in blind ends. Their function has been much disputed, 

 some thinking them organs of secretion, and others organs of 

 excretion that is, some suppose their products are needful for 

 the better digestion of tho food, while others think it worthless 

 and injurious if not thus got rid off. The first-named function 

 is performed by the liver in vertebrates, and the last-named 

 by the kidney. The names of these organs have, therefore, been 

 applied to these blind tubes by the advocates of the respective 

 views. Such names are liable to confuse the reader, as there 

 can be no correspondence between organs taken from different 

 sub-kingdoms, so far as structure is concerned. Hence, a new 

 name has been applied to them, which is not liable to mislead, 

 that of Malpighian vessels. Below the entrance of these Teasels, 

 the alimentary canal contracts, and has been called the small 

 intestine. At the end of this is a valve which prevents an 

 entrance of matter from behind. Beyond the valve is the largo 

 intestine, which sometimes dilates into a chamber into which 

 the reproductive products and the poison from tho poison-bag, 

 where it exists, empty themselves. The orifice of exit is always 

 at the extreme end of the animal. It should have been men- 

 tioned that into tho mouth of the oesophagus two or more 

 salivary glands usually empty themselves. These are often more 

 or less attached to the sides of the canal, but are sometimes 

 free and floating in the juices of the body. Sometimes the liquid 

 secreted by these is very pungent and irritating, though why it 

 should be so it is difficult to conjecture, unless, by being poured 

 into the wound made by the compound lancet, it causes a flow 

 of blood to that part. Otherwise, one would have supposed that 



tin' irritation caused would hare been equally disturbing to the 

 M g insect and the victim of its attack. 



One of the greatest peculiarities of insects though, as wo 

 have seen, the apparatus is not confined to them is the trachtal 

 system. In insects the necessary process of the aeration of the 

 blood is not accomplished through a soft membranous skin, for 

 this in them is hard and dry ; nor by protrusions of the circu- 

 latory system, so as to expose the contained blood to the ****TmT 

 of the surrounding medium ; nor even by setting apart son* in- 

 ternal cavity for the process ; but the air is introduced and teat 

 by small dividing and sub-dividing vessels into every organ of the 

 body, and so the function of respiration is diffused through all 

 parts. The entrance of the air to the body is not through the 

 mouth, as in vertebrate animals, but through a number of oral 

 holes in the sides of the body. As a rule, it might be said that 

 there are a pair of these to each segment of the body, but they 

 are by no means always present in every segment. These oral 

 holes are called spiracles, or breathing pores. They are well seen 

 in the illustrations of the caterpillar and chrysalis of the privet 

 hawk-moth. Their presence in the perfect moth would be ob- 

 scured by the thick coating of scales and hairs, even if the side 

 riew were given. These orifices need some protection, especially 

 at certain times, and thus they are often placed in situations 

 where they cannot readily be seen. In the abdomen of the 

 beetle they are often placed under the deck-like fore-wings. In 

 the bees and wasps they are found in the side-pieces, which 

 are covered by the back and belly-plates of the rings. Beside* 

 these methods of protection, they are often defended by a thick 

 fringing of hair, or by plates, or by membrane. These organs 

 at the spiracles are sometimes vibratile, and the humming of 

 insects is sometimes thus caused. 



In the sketch of 'the trachea! system of a species of parasite 

 given in the picture, it will be seen that each spiracle has a little 

 globular enlargement below it, and that canals lead from these 

 to join a large lateral canal which runs down the side of the 

 body, uniting with its fellow both before and behind. From 

 certain parts of this similar vessels are given off which run to 

 the more important parts of tiie body, and there break up into 

 branches. In this creature the trachea! system is of the simplest 

 kind, but in the generality of insects it is very complex, the two 

 Literal canals sending off cross branches, while from all parts 

 of the main canals branches spring, and by dividing and sub- 

 dividing, run into all the organs, so that no small portion of the 

 viscera, whether taken from the nervous or any other system, 

 can be examined under a microscope without revealing some of 

 these vessels. Indeed, it would seem that, united with the 

 function of respiration, they perform the additional one of 

 sustaining the internal organs. 



It may be asked, how can these minute vessels be distin- 

 guished from others when under a microscope P This is easily 

 done on account of the peculiarity of their structure. Each 

 tube consists of two thin membranes, with a spiral thread lying 

 between them. The membranes are transparent, and the closely 

 coiled thread within them looks like the wire spring of a bell. The 

 use of these spiral threads is manifest. The air tubes must be 

 kept open in order that respiration may be carried on, and yet 

 tho movement of the body, or any pressure, is liable to close 

 them. These elastic coils, therefore, maintain the tube, and by 

 their resiliency open it when it has been closed by pressure. 



The nervous system is quite after the type of all Articulates, 

 which we have described elsewhere. 



One of the most striking and interesting of phenomena in 

 nature is the transformation at certain stages of the develop- 

 ment of insects. Such transformations ore not confined to this 

 class, but they were first noted in it, and are better denned and 

 studied in insects than in any other class. Most animals have 

 a protective epidermis, which, being extra- vascular, is dead or 

 dying. Some have this constantly wearing off, while others 

 retain it for a time, and slough it off atone act. Many creatures 

 also, during growth, go through considerable change in the pro- 

 portion and structure of these parts. Now, in insects, since the 

 whole form of the animal is dependent on the external integu- 

 ment, and the whole of this is thrown off together, while at 

 the same time extreme changes have gone on in the internal 

 structure, we have, as a result, periodical changes of so radical 

 a character that they are called metamorphoses. These changes 

 are, however, very different in different insects. In all there is 

 a growing state, in which they eat enormously. This state is 



