FORM, GROWTH, AND CHANGE 35 



(or ecdysis), the skin separates from the cuticle, a fluid secreted 

 by cells of the skin occupying the intervening space. On the 

 surface of the skin a new cuticle begins to form and, while this 

 has generally the structure of the old, new features may be 

 introduced by outgrowths of various regions of the body, 

 which being covered by skin will have the outermost coating 

 or cuticle formed over them. Thus during a new stage in 

 the life-history, the insect may differ appreciably, if slightly, 

 from its condition in the preceding stage. For the accom- 

 plishment of each moult, the cervical bladder, between the 

 head and prothorax, already mentioned as effective in the 

 processes of hatching and casting of the amnion, is again 

 brought into use. By the pressure of this bladder the dorsal 

 cuticle is split lengthwise and through this split, the insect, 

 clad in the new cuticle which has formed beneath the old one, 

 carefully extricates its body and limbs. First the head and 

 prothorax are withdrawn, then the feelers, then successively 

 the first and second and third pair of legs, and finally the 

 abdomen. The new instar (a convenient term for an insect 

 in any one stage of its life-story) is larger than the former one, 

 but the body within the old cuticle has been contracted and 

 the skin folded. Now by swallowing air and increasing the 

 blood-pressure, not only is the old cuticle split, as we have 

 seen, but the newly-emerged instar expands, and it follows 

 that the new cuticle when through deposit of the lower 

 " secondary ' layer beneath the first-formed " primary " 

 sheet (see Fig. 2) it has become in the exoskeletal parts, 

 thick and firm, is found to be more extensive than the old. 

 Thus through its succession of moults the insect grows in 

 size and may also change in form. 



The second instar of the grasshopper (Fig. 19 b) shows 

 increase in bulk as compared with the newly-hatched insect, 

 but no marked change in structure. There is as yet no trace 

 of wings, and the thoracic segments are still closely similar, 

 though the foremost (prothorax) shows a slight relative 

 increase in length when compared with the other two. After 

 the second moult, when the third stage (Fig. 19 c) in the 

 life-history has been reached, the prothorax begins to assume 

 the prominent hood-like aspect which characterizes it in the 

 adult, and the wing-rudiments on the mesothorax and meta- 



