94 EMBRYOLOGY OF INSECTS AND MYRIAPODS 



dibular glands occur in many insects; in the case of their appearance in the 

 larvae they begin their development in the embryo. In Calandra callosa, 

 Wray (1937) found the invaginations on the inner side of the rudiments 

 of the mandibles and somewhat between them and the base of the 

 maxillae. Maxillary glands occur in some apterygotes as well as among 

 pterygotes. In Forficula, Heymons (1895a) described an epidermal 

 invagination at the base of each of the first maxillae from which an 

 irregular system of sacs and tubes develop. They seem to correspond to 

 certain of the head glands of the Chilopoda. The glands that are usually 

 most highly developed in insects are those of the labium. Since they 

 are of variable function, the term "labial glands" is preferable to the 

 usual designation of salivary glands. In the embryo they originate as 

 paired invaginations of the ectoderm just behind the bases of the rudi- 

 ments of the second maxillary appendages. As development progresses, 

 the two orifices unite medially on the venter of the second maxillary 

 segment. At the same time, the appendages of this segment also come 

 together and unite on the middle line. The median orifice of the glands 

 having moved forward, it comes to lie in the ventral wall of the head 

 anterior to the base of the labium. Although typically these glands are 

 simple or convoluted tubes, they may be branched or provided with 

 terminal vesicles. In the larvae of the Lepidoptera, Trichoptera, Coleop- 

 tera, Siphonaptera, and most nemocerous Diptera and Hymenoptera 

 the glands are silk-producing structures. 



In the moth Diacrisia on the second maxillary segment, in addition 

 to the invagination for the silk gland, there is an outer pair which will 

 form the so-called " hypostigmatic gland " of Toyama (1902). This gland 

 develops as an invagination near the root of the second maxilla (labium). 

 In the embryo at the time the mid-gut rudiment appears, the invaginated 

 cells are larger than those adjacent, the entire invagination spherical in 

 form and gland-like in appearance, with the mouth of the invagination 

 directed cephalad. Shortly afterward the structure has migrated mesad 

 and slightly caudad, coming to lie in front of the prothoracic legs, and has 

 lost its attachment with the surface. Later, its cells can no longer be 

 distinguished from those of the adjacent mesoderm. Nelson (1915) 

 suggests that this structure may represent the corpora allata, though we 

 are inclined to believe that it is the ventral cervical gland, embryonic in 

 this species but highly developed in the caterpillars of Schizura concinna, 

 Dicranura vinula, and other notodontids though vestigial in many other 

 lepidopterous larvae. In the silkworm (Toyama, 1902) it persists in the 

 larva as branched glands in the first thoracic segment. It may be 

 homologous to the first thoracic Gilson's gland of the Trichoptera. 



Heymons (1895a) has called attention to two bodies found in Forficula 

 and Gryllus, which he named "ganglia allata." Later (1899) he dis- 



