1922] Uichanco — Notes on Parthenogenetic Macrosiphum 69 



future dark pigments are deposited in the form of certain achro- 

 matic substances and in the succeeding instar assume their 

 characteristic tint upon exposure to hght and to other external 

 stimuli. 



Janet (1909) has classified in a succinct form the steps in- 

 volved in ecdysis among insects into four general successive 

 phases. The following quotations from this author are repro" 

 duced here for the purpose of comparison and correlation with 

 the foregoing observations and deductions on aphids. 



1. "Le decollement de la cuticule ancienne, decollement 

 qui commence generalement dans la region superieure de la cap- 

 sule cephalique et se propage vers I'arriere. La cuticule ancienne, 

 decollee, demeure autour du corps sous forme d'une exuvie libre, 

 intacte, et constitue une enveloppe protectrice, momentanement 

 indispensable a cause de la delicatesse de I'epiderme mis a nu." 



2. "Accroisement en surface ou transformation du modele 

 de I'epiderme libere de sa cuticule." 



3. "La formation des premieres strates de la cuticule nou- 

 velle, cuticule dont I'apparition rend inutile la protection que 

 fournissait I'ancienne cuticule exuviee." 



4. "Le dechirage et le rejet de I'ancienne cuticule." 



It appears in the case of the aphids, as well as of other in- 

 sects, that the above four steps overlap, one into the other. 

 During the detachment of the old cuticle from the insect's 

 body-wall, there undoubtedly occurs simultaneously an increase 

 in surface area of the hypodermal layer. In fact, the stretching 

 effect resulting from the latter cause, together with the steadily 

 augmenting pressure from the developing internal organs, evi- 

 dently provides the immediate mechanical means for separating 

 the old chitinous from the hypodermal layer. Under the protec- 

 tive cover of the freshly loosened exoskeleton, the new chitinous 

 layer which has been secreted by the hypodermal cells has a 

 chance to develop a firm, although somewhat delicately soft, 

 consistency before the initial rupture appears in the exuviae. In 

 aphids, the hardening of the newly formed exoskeleton occurs 

 mainly during the lengthy process of casting off the old skin. 



Summary of Section I. Feeding ceases a few minutes prior 

 to molting. The head of the insect is pointed toward the ground 



