that in which it spends most of its time, eating and growing, is the larval stage. So true is this, that in many insects whole families sometimes, especially among the lepidoptera and May-flies, the digestive systems and mouth parts in the imago are so far aborted as to become wholly useless for feeding. Again, in others, as in the elmleaf beetle, where the insect hibernates through the winter in the imago form, food is taken in fall solely to store up fat for winter use, and in the spring only until sexual maturity is reached, when the insects mate, oviposit and die. Returning to our butterfly, we find the mouth parts formed into a long proboscis, coiled up at rest, but capable of being thrust to the depths of the flower-cup to sip the nectar or honey found therein. Leading from the mouth through the thorax is the long, slender œsophagus, which divides, one portion leading into the food-reservoir, the other into the stomach. The food-reservoir corresponds in function to the crop as a place of storage, though instead of being a simple dilation of the canal through which all food must pass, it is a sort of bladder-like outgrowth to contain only that portion of food which cannot make its way directly to the stomach-the surplus, in other words. The chylific ventricle is a straight tube of considerable capacity, but lacking entirely any cæca or gizzard-like structures at its entrance, as the absence of solids in the insect's food renders their presence unnecessary. The usual malphigian vessels are present in great numbers, being simple hair-like tubes. The intestine is of rather small diameter, coiled at least once and ending in a good-sized, pear-shaped colon. The excrement is very small in quantity and pasty in consistency. The salivary glands are well developed, consisting of two paired, thread-like bodies opening into the buccal cavity, having but one orifice to each pair. We next consider the fly, our sketch being made from a blow-fly, Calliphora vomitoria. Fig. IV. Flies feed in one of two ways, either by piercing and sucking or by lapping and sucking; the fly in question being one of the latter sort, as the galea are formed into a lapping and scraping organ. As they feed to a large extent upon solids, such as sugar and other substances, means must be provided to reduce these to a liquid state by solvents before they can be taken into the mouth. Hence we have a very large development of the salivary glands, and a correspondingly large secretion of saliva for this purpose. The glands are thread-like and coiled into a tangled mass. There is also a sort of bladder for the storage of the saliva. The œsophagus is long and slender, as in the preceding species; the crop a dilation directly upon the canal itself, not like the outgrowth in the butterfly. The chylific ventricle is not very long, and of small capacity compared with those of our preceding species. There is no evidence of either cæca or gizzard, but the malphigian glands are well developed, perhaps not so numerous as those we have seen, but larger, and resembling those of the caterpillar with their bead-like dilations. The intestine is long, coiled, but simple in structure, and opens into a curiously-shaped colon. Fig. VII. The colon is somewhat rectangular in shape, with four large lobes, one at each corner, but inclining from the anal extremity. The rectum is longer than usual, being fully equal in length to the colon, and its excrement is similar to that of the butterfly. In either case the material hardens almost as soon as voided. Lastly we have the digestive system of the bumble-bee, Bombus sp., and here we find the greatest length and sharpest differentiation into separate regions. Among the Hymenoptera, to which order the bees belong, we find not only the haustellate, but mandibulate type represented as well, hence we should expect to and do find traces of characteristics of both. Here again we find the long œsophagus extending through the thorax, which indeed seems to be the rule; the exception in the case of the grasshopper being due to the great development of the reproductive organs, which at maturity fill almost the entire abdomen, thus forcing a portion of the alimentary canal into the thorax. The crop in Bombus is quite large, membraneous in structure and readily dilated with chitinous, plate-like structures bearing longitudinal ridges at its insertion into the stomach. This structure is evidently a gizzard, morphologically speaking, yet as liquid food is generally taken, it is in this case apparently functionless. The stomach is large, exceeding the capacity of the crop, with a curious differentiation in the size of its transverse striations at the anterior end. There is a total absence of cæca; but there are an immense number of malphigian vessels almost entirely enveloping the canal as it lies closely coiled in the insect. The intestine is of small diameter, but equal in length to the stomach, and ending in a membraneous, distensible colon. The salivary glands are fairly well developed, and have only the usual digestive function. In summing up the characteristics of those insects we have classified as mandibulate in contradistinction to the haustellate type, we can only state general facts which seem to meet with many exceptions. The presence of gizzard and cæca seems to indicate the mandibulate character, as they are absent in the true haustellate type. The canal, as a whole, seemed to be shorter and straighter in the case of mandibulate insects, hence of lower type than the haustellate forms. The salivary glands were well developed in both divisions, though in the haustellate insects used solely for digestive purposes, while in the mandibulate forms with additional functions, one of cocoonbuilding or protection and the other of defense. There is in the haustellate type in some instances a modification of the pharynx, the so-called sucking stomach, by whose aid liquids are drawn into the mouth. In the case of the mandibulate forms we may find in the receum certain moulding appliances, which give to the pellet of excreta a characteristic form by which often the insect may be determined, while in the haustellate type no such modification exists, as no impression could be left on liquid excrement. |