CHAPTER VIII 



ECTODERMAL DERIVATIVES 



THE INTEGUMENT 



In the later embryonic stages the epidermis secretes at its surface a 

 cuticula which in some cases may be thin and dehcate but in other cases, 

 as in the head capsule of many holometabolous insects, may be relatively 

 thick. This cuticula may be shed soon after hatching except in holo- 

 metabolous forms. Hairs, setae, and scales arise from large epidermal 

 cells (trichogens) which in some instances are very conspicuous. In the 

 late embryo of certain moths {Diacrisia and others) they are by far the 

 largest of the ectodermal cells, extending far below the level of the neigh- 

 boring cells. The setae, scales, and other structures are formed on the 

 attenuated, slender, cytoplasmic processes of the trichogens. Other 

 epidermal cells, either singly or in groups, sometimes deeply invaginated 

 below the surface of the body wall, form the epidermal glands of various 

 sorts. 



THE ENDOSKELETON 



A system of apodemes — inward invaginations in the form of rods or 

 ridges — is developed for the attachment of muscles and for the support 

 of internal parts. In the head these processes consist of an anterior and a 

 posterior pair of arms w^hich are connected by the transverse bar-like 

 body, together forming the tentorium. The anterior tubular invagination 

 develops on the inner side and somewhat in front of the mandible or 

 behind and near the base of the antenna. The posterior invagination 

 arises behind the base of the first or between the bases of the first and 

 second maxillae. From the posterior arm a transverse branch arises 

 which meets its fellow from the opposite side, thus forming the body of the 

 tentorium. In some insects a pair of dorsal arms are formed as diverticula 

 of the anterior pair. According to Strindberg (1913b) two transverse 

 connecting bars instead of one represent the body of the tentorium in 

 Euterm.es. Epidermal invaginations constitute the apodemes for the 

 attachment of muscles and of the appendages in the head, thorax, and 

 abdomen. 



GLANDS 



Antennal, mandibular, maxillary, and labial glands all arise as sac- 

 like invaginations of the epidermis. Antennal glands have been described 

 for the roach which presumably originate during embryonic life. Man- 



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