294 NATURAL HISTORY. 



suspensory ligaments. They are cylindrical membranous tubes, within which there is distinguishable 

 a tine annulation, resembling a spiral chitinous thread, and this structure, which is continued 

 into the very finest ramifications of the tracheae, serves to give them sufficient elasticity to remain 

 constantly open for the free ingress of the air. In general the wide tracheae, which start directly from 

 the stigmata, run inwards but a short distance, and then open into a longitudinal vessel of the same 

 kind, which passes \ip the side of the abdomen, uniting all the main stems, and in this way a pair of 

 lateral longitudinal tracheae are produced, from which the smaller branches going to the various organs 

 are given off ; but occasionally the tracheae from the stigmata run directly into the body. In many 

 insects bladder-like dilatations of parts of the tracheal system occur, and these sometimes form very 

 large air-sacs in the interior of the body ; they are of membranous texture, and destitute of the spiral 

 thread, although this makes its appearance again in the fine branches given off from them. Special 

 modifications of this respiratory apparatus are, however, frequent in insects. In many cases, 

 especially in air-breathing aquatic larvae, the function of the stigmata of the sides of the body is 

 suppressed, and the apertures themselves closed up, and respiration is effected solely by the agency 

 of peculiarly modified stigmatic apertures at one or other extremity of the body. Again, many 

 aquatic larvae dwell constantly in the water, never coming to the surface to breathe, and, in these, 

 while the structure of the tracheae remains the same, we find, in place of the stigmata, peculiar organs, 

 which have been called tracheal gills, by means of which the insects respire the air dissolved in the 

 water they inhabit. These gills are usually leaf-like organs containing branched tracheae, and they ai - e 

 sometimes appended to the sides of the abdominal segments, or confined to its posterior extremity. 



Reproduction in insects takes place usually by eggs, which are deposited in suitable situations 

 by the females. In many cases they are merely attached singly or in groups to plants or other 

 objects, or deposited in the ground ; in other instances they are inserted into the substance of 

 the plant or animal on which the larvae feed, by the agency of a peculiar organ (ovipositor) with 

 which the female is endowed for this purpose ; and sometimes the parent insects prepare nests of the 

 most complicated character for the reception of their eggs and the subsequent rearing of their 

 offspring. In some cases, however, the development of the eggs takes place within the body of the 

 mother, and instead of eggs larvae are then brought forth. A few insects ven go farther than this 

 and retain the larvae within their bodies until they have arrived at maturity, producing their young in 

 the pupa state. These, however, can only be regarded as exceptions to the general rule, according to 

 which the eggs are deposited before any development of the larvae has taken place within them, and 

 impregnated during their passage outward from the ovary through the oviduct, by contact with the 

 male fertilising element, which has been stored in a special receptacle appended to the oviduct since 

 the union of the sexes. The last-mentioned point is one of considerable importance in connection with 

 the phenomenon of the production of insects from unfecundated eggs (parthenogenesis), and especially 

 in the explanation of the constitution of certain societies of insects (such as Ants, Wasps, and Bees). 

 In these it appears to be proved that the male individuals are produced from unfertilised eggs ; and in 

 a number of other insects eggs in the same condition have been known to produce larvae, whilst 

 of some no males have ever been seen, although the insects have been bred for several generations. 



In one remarkable group of insects, including the Aphides, or Plant-lice, and some allied forms, 

 reproduction takes place in a peculiar manner, which has been called parthenogenesis, but is really 

 analogous to the so-called " alternation of generations," so frequent among animals much lower in the 

 scale of organisation. In these insects, true male and female forms appear at certain intervals, 

 and the latter produce true eggs; but between the hatching of these and the production of the 

 next true males and females among their progeny several generations of insects succeed one another, 

 which bring forth young by a process analogous to internal budding. The result of this process is 

 sometimes a young living insect, sometimes a more or less egg-like body, and the history of the 

 reproduction of these little creatures is thus rendered exceedingly complicated. 



We have now only to indicate briefly the classification that will be adopted in the following 

 pages. By going back over the preceding statements the reader will find that there are two sets 

 of characters, by either of which the class of insects may be divided into two great sections, namely, 

 the characters drawn from the structure of the mouth, that is, whether this is adapted for mastication 

 or for sucking, by which we get the two groups of mandibulate and haustellate insects ; and those 



