236 NATURAL HISTORY. 



jointed organ, the maxillary palpus ; and these two parts are regarded by zoologists as representing 

 the shank and the tarsus. But besides these parts, the stein-joint bears on its inner side two 

 masticating-plates, which are generally horny, variously-toothed, and assist in the division of the food. 

 This, of course, is a very general description of the structure of the maxillae, which present many 

 varieties of formation in different members of the class Insecta. 



Below or behind the maxilla? we find what is apparently a single organ -the lower lip, or 

 labium but which is really composed of a pair of organs united together in the middle line. The 

 labium closes the mouth from below, and consists of several parts, which have received special names. 

 Thus its basal part, which represents the two hinge-joints of the maxillae amalgamated together, is 

 known as the chin, or mentum ; the part in front of this, which may be either horny or membranous 

 in texture, is called the ligula, and corresponds to the stem-joints, and other parts of the maxilla?. It 

 is not unfrequently cleft in the middle, and may also have one of the other portions of each half 

 separate, forming distinct divisions, called paraglossce. The labium also bears a pair of palpi (labial palpi). 



The preceding statements are intended solely to give a general idea of the arrangement of the 

 parts composing the mouth in ordinary insects, to clear the way for the descriptions of those modi- 

 fications of structure which, it will be seen hereafter, are of great importance in the classification of 

 these animals. They are of importance, also, in connection with the theoretical structure of the 

 head, which, although apparently composed of one solid piece, must be regarded as really consisting of 

 several segments, intimately united to one another. If we consider what these segments may be, and 

 what is their theoretical number, interpreting the insect head by its homologies with the lower 

 arthropods, we find that the following constituents may fairly be distinguished, a segment beai-ing 

 the eyes ; a second segment, bearing the antennae ; and three more, of which the mandibles, maxilla?, 

 and labium are appendages ; the mandibles, which are solid, and bear no palpi, being regarded as 

 representing only the basal joints (coxae) of the members belonging to their segment. We thus get 

 five segments in the head, which, taking eleven as the full number of segments in the abdomen, 

 would give nineteen as the total number of segments in insects ; but some anatomists are inclined to 

 think that a second antenna-bearing segment must be imagined to exist, although in an undeveloped 

 state, in order to bring the number of segments into accordance with that present in Crustacea. A 

 remarkable and. so far as we know, perfectly exceptional, structure was described in 1879 by 

 M. H. de Saussure, in a small Cockroach-like insect (Hemimeru-s), from the Gaboon, on the West 

 coast of Africa. In this curious creature M. de Saussure finds a second labium, evidently composed of 

 two halves, and having a pair of palpi, situated within the regular labium, and between it and the 

 maxillae. This, of course, would make the number of head-segments six, without reckoning the hypo- 

 thetical second antennal segment, but unfortunately it is in the wrong place, and its occurrence is so 

 exceptional that M. de Saussure is inclined to remove the creature possessing this remarkable 

 character altogether from the class of Insects. 



The modifications which these parts undergo, and which are characteristic of the orders, and other 

 groups of insects, are very considerable, and will have to be fully described farther on ; but a brief 

 statement of the nature of the more important of them will not be out of place here, as placing the 

 very curious phenomena in question before the reader in a connected form. The description given 

 above indicates the general arrangement of the parts in the mouth of ordinary biting insects ; and the 

 differences presented by these are generally in matters of detail, such as the relative proportion of 

 parts, &c. The first type which requires notice here is that presented by the Bees, in which the 

 horny mandibles still retain their ordinary form and arrangement, and are, indeed, most efficient 

 biting organs ; whilst the rest of the organs of the mouth undergo important changes to fit them for 

 the sucking-up of fluid nutriment. For this purpose the mentum, or basal part of the lower lip, 

 acquires considerable power of movement, and the ligula, attached to it in front, becomes greatly 

 elongated, at the same time that the maxillae, which are also much elongated, acquire the form of thin 

 blades, which embrace the sides of the ligula. By the union of these parts a sort of tube is formed, 

 through which the food of the animal, consisting of the honey of flowers, can be easily sucked up, the 

 mode of articulation of the parts enabling the whole composite organ to be pushed forward, or 

 retracted beneath the head, at the will of the animal. Both labium and maxillae are still provided 

 with palpi. 



