Page:Encyclopædia Britannica, Ninth Edition, v. 13.djvu/155

INSECTS 143 more than passing notice bestowed upon it. Modern scientific travellers have not succeeded in confirming Madame Merian s well-known statements with regard to the luminosity of the so-called lantern-flies (Fulgora), hence these have to be eliminated from the category of luminous insects. It is among the Coleoptera that the phenomenon especially occurs, and in them is almost confined to certain skip-jack beetles (the genus Pi/rophorus), and probably the entire family of glow-worms (Lampyridee). The luminosity is confined to certain dis tinct patches, differing in position and number according to the species and also according to sex, usually most observ able in the female, although this does not appear to be always the case. The property is distinctly under the control of the insect, and is often exercised in an inter mittent manner at stated intervals, when the insects are not under the influence of extraordinary excitement. It is probable that luminosity exists in some Diptera, and also in the larvae of certain exotic Lepidoptera, a recent traveller having assured us that in South America a larva of this order has luminous patches along either side, so that when in motion it has been compared to a lighted-up railway train. Some occasional instances of luminosity appear to be accidental, probably owing to the insects having been feeding upon, or otherwise in contact with, decaying phosphorescent matter. With regard to the nature of the luminous substance no very precise results have been arrived at by investigators. That it is phos phorus in some form or other appears certain, and the latest experimenter (Jousset de Bellesmc) asserts as his belief that it is no other than phosphoretted hydrogen gas stored up in the cellular tissue, and in direct communica tion with the nervous and respiratory systems. Galls. These well-known insect-productions are alluded to chiefly in order to call attention to the mystery that surrounds their growth. Galls are occasioned by the presence of the larvae of certain species in nearly all orders of true insects, though it is amongst the Cynipidx in Hymenoptera and the C ecidomyiidse, in Diptera that they are most characteristic. And they may be in almost any position on a plant, according to the species of gall-maker. The most striking, however, are clearly modified leaf or flower-buds. The mystery surrounding galls is their cause. The in direct cause is the puncture of the insect, and the presence of its eggs or larvae, but no explanation has been offered of the reason why this presence sets up the growth termed a gall. Two insects of differing species will deposit their eggs in the same position : in the one case no abnormal growth follows ; in the other some peculiar irritation sets up a tumour, often enormous in size. Two insects, also of different species, but both gall-makers, do the same : in both cases a tumour ensues, but its form is totally different in the two. A most noticeable recent discovery is that by Dr Adler (since confirmed by others) to the effect that in certain European Gynipidse, dimorphism to a remarkable extent occurs, and that certain genera are only conditions of others, the two forms of insects, and the totally different galls occasioned by them, being alternate in appearance. External Structure. Taking any large insect, we recog nize in it three more or less distinctly separated divisions, the head, thorax, and abdomen. Taking the majority of insects, and especially of their larvae, we recognize thirteen segments or somites, counting the head as one, the thorax as consisting of three, and the abdomen as nine. From a classificatory point of view, it is probably convenient to retain this idea, though in the abdomen of a dragon-fly (for instance) there are 10 quite distinct segments. But, seen in the light of embryology and morphology, a different aspect is put on, The abdomen in the embryo of some insects clearly consists of 11 segments. Moreover if each appendage of the head be considered as a modified limb, we get in some insects as many as 7 segments in this portion of the body alone. Thus although 13 segments is a usual and convenient number as regards the structure of an insect, this number must be vastly increased if we consider the animal in regard to other divisions of the Arthropod series. In the abdomen the actual number is sometimes very much reduced, owing to several of the segments becoming obsolescent, coalescent, or retracted. The exoskeleton, or outer covering, is more or less horn like in its nature. But its elements are by no means similar to those of either horns or bones. It is composed to a varying extent of phosphate of lime, with the addition of a peculiar substance termed chitine, especially character istic of, though not strictly confined to, the Insecta. According to recent analysis, the constituents of chitine are said to be as follows : Carbon 45 69 Hydrogen 6 "42 Nitrogen 7 00 Oxygen 40 89 The head, or anterior of the three main divisions of the body, of a perfect insect is of very varying form and structure, both as regards outline, the condition of its attachment to the trunk, and the details of its special appendages. No account of these variations can be given here ; they will be briefly alluded to in the classificatory portion of this article. The various organs and appendages may be stated as follows. On each side of the anterior portion are inserted two long and usually multi-articulate processes termed "antennae," which are tubes containing nerve-prolongations and trachea. , and undoubtedly associated in a high degree with the special senses; but, notwithstanding all the controversy on the subject that has existed and still exists, we do not yet know clearly what is their special function. They have been considered organs of touch, of hearing, of smell, or simply as balancers assisting and directing flight. No one who has watched the proceedings of many insects (and especially of ants), when meeting others of their kind, can doubt that they act in some way (but perhaps not in all insects) as means of intercommunication, and thus take a high rank as important structures. They are, as a rule, much less developed in those insects having very large eyes ; and in the larvae of those that undergo a complete metamorphosis they are usually rudimentary only, notwith standing their often enormous development in the perfect insects produced from the same larvae. The compound eyes are two in number (though each is occasionally divided into two portions), usually of large, sometimes of enormous size, and each consisting of very numerous facets, which but indicate the faces of so many independent angular tubes separated by layers of pigment. In the larval state the eyes are ordinarily simple, and each eye is usually a congregation of separate eye-spots. Besides the compound eyes, there are two or three (or no) small simple eyes, "ocelli"or "stemmata," each with a simple nerve, and never present in the lame or (probably) pupae. The organs of the lower surface of the head are of a most complicated nature, and are excessively modified according as the insect takes nutriment by biting or by sucking. Below the eyes is the " front "; this is succeeded by a piece termed the "clypeus"(or "epistome" or "nasus"), which is followed by the "labrum" or upper lip. On either side are the "mandibles" (usually dentate within) articu lated to the cheeks, and below these a second pair of jaw.s, compound in structure, and consisting of a hinged base, afterwards frequently dividing into two portions, the "maxillae" and maxillary lobes, and provided externally with articulated appendages known as the maxillary palpi. Below the mouth is the "labium" with its labial palpi, articulated to the " mentum " or chin-piece ; lying within this lower