can, however, be very frequently actually demonstrated, so that it is correct to speak of ten as the normal number. {88}[Illustration: FIG. 47—Diagram of exterior of insect: the two vertical dotted lines indicate the divisions between H, head; T, thorax; and A, abdomen: _a_, antenna; _b_, labrum; _c_, mandible; _d_, maxillary palpus; _e_, labial palpus; _f_, facetted eye; _g_, pronotum; _h_, mesonotum; _i_, metanotum; _k_, wings; _l_{1}_ to _l_{10}_, abdominal segments; _m_, the internal membranous portions uniting the apparently separated segments; _n_, cerci; _o_, stigma; _p_, abdominal pleuron bearing small stigmata; _q_{1}_, _q_{2}_, _q_{3}_, pro-, meso-, meta-sterna; _r_{1}_, mesothoracic episternum; _s_{1}_, epimeron, these two forming the mesopleuron; _r_{2}_, _s_{2}_, metathoracic episternum and epimeron; _t_, coxa; _v_, trochanter; _w_, femur; _x_, tibia; _y_, tarsus; _z_, gula.] It is no reproach to morphologists that they have not yet agreed as to the number of segments that may be taken as typical for an Insect, for all the branches of evidence bearing on the point are still imperfect. It may be well, therefore, to state the most extreme views that appear to be at all admissible. Hagen[18] has recently stated the opinion that each thoracic segment consists really of three segments—an anterior or wing-bearer, a middle or leg-bearer, and a posterior or stigma-bearer. There seems to be no reason for treating the stigma as being at all of the nature of an appendage, and the theory of a triple origin for these segments may be dismissed. There are, however, several facts that indicate a duplicity in these somites, among which we may specially mention the remarkable constancy of two pleural pieces on each side of each thoracic segment. The hypothesis of these rings being each the representative of two segments cannot therefore be at present considered entirely untenable, and in that case the maximum and minimum numbers that can be suggested appear to be twenty-four and eleven, distributed as follows:— Maximum. Minimum. {89} Head 7 3 Thorax 6 3 Abdomen 11 5 -- -- Total 24 11 Although it is not probable that ultimately so great a difference as these figures indicate will be found to prevail, it is certainly at present premature to say that all Insects are made up of the same number of primary segments. A brief account of the structure of the integument will be found in the chapter dealing with the post-embryonic development.