12 INSECT TRANSFORMATION 



these lobes are so strongly developed (Fig. 4 B) as to suggest that 

 they represent a pair of jaws known as the maxillulae lying 

 between the mandibles and the maxillae. Further it has been 

 shown that, in some insects, the embryo, as it develops in the 

 egg, has a segment with a pair of rudimentary limbs between 

 the feelers and the mandibles. Thus the conclusion is reached 

 that the insectan head may contain not five segments only, 

 but seven 1 including that to which the labium belongs. Such 

 studies are of interest, because they throw light on the history 

 of structures which undergo progressive changes during the 

 evolution of the race. It seems that the compact head of a 

 grasshopper or a beetle has arisen from the association of seven 

 segments, originally distinct, each one bearing its pair of appen- 

 dages ; of these seven, the third and fifth have now become so 

 reduced as to be hardly recognizable, while the seventh was, 

 so to say, annexed by the head after the others had been incor- 

 porated into the primitive head-skeleton. The composition 

 of the insectan head has been explained with some detail 

 because the parts are often strangely modified during the early 

 stages of an insect's life. 



By means of a flexible neck probably representing the 

 segment of the labium the grasshopper's head is joined to the 

 great central region of the body, the thorax. This is composed 

 of three segments known respectively as the prothorax, meso- 

 thorax, and metathorax, and in each segment may be distin- 

 guished various sclerites which build up the tergal surface 

 above, the sternal surface beneath, and the pleural areas at 

 the sides. The tergal sclerite of the prothorax is extensive, a 

 firm hood-shaped plate distinguished as the pronotum (Fig. 9 pr.) 

 which reaches backwards so as partly to overhang the seg- 

 ments behind. In each of these a series of sclerites, whose 

 boundaries are marked by grooves in the cuticle, may be dis- 

 tinguished from before backwards these are defined as pre- 

 scutum, scutum, scutellum, and post-scutellum (Fig. 5 Psc, Set, 

 Scl, Pscl). We notice that the scutellum is subtriangular in 

 shape, the apex directed backwards ; in some insects the meso- 

 thoracic scutellum is relatively much larger and more conspicu- 



1 J. H. Comstock : " An Introduction to Entomology". Ithaca, New 

 York, 1920. J. H. Comstock and C. Kochi : "The Skeleton of the Head of 

 Insects". Amer. Nat., XXXVI. 1902. J. W. Folsom : "Entomology". 

 Philadelphia and London, 1906. 



