258 EMBRYOLOGY OF INSECTS AND MYRIAPODS 



The blastoderm is composed of uniformly deep, nearly cubical cells. 

 With the formation of the germ band, which extends from the posterior 

 end of the egg to beyond the middle on the ventral side, the remaining 

 blastoderm cells become thinner. The invagination of the germ band 

 occurs near the caudal end of the egg. The inner layer is formed on both 

 the germ band and the amnion when the germ band is partly invaginated. 

 Later the inner-layer cells which have been formed on the yolk side of the 

 amnion are liberated as paracytes which later degenerate in the yolk. 

 The yolk has meanwhile undergone a secondary cleavage into yolk spher- 

 ules, as described for Coleoptera, Lepidoptera, and some other insects. 

 When the caudal end of the embryo has invaginated nearly to the anterior 

 end of the egg, it becomes strongly flexed ventrally. Before the head of 

 the embryo has lost its connection with the periphery, the large head 

 lobes have developed. Stomodaeum and proctodaeum appear after the 

 embryo is completely immersed in the yolk. At the blind end of the 

 proctodaeum two buds appear which represent the anlagen of the four 

 Malpighian tubules. Strindberg states that the tracheal invaginations 

 appear on the prothorax and on abdominal segments two to seven. 



Coelomic sacs are formed by the bending over of the outer margin 

 of the mesoderm segments. In all there are 19 pairs, of which that of the 

 tritocerebral segment is smallest and lacks a cavity. According to 

 Strindberg it develops into the subesophageal body which later appears 

 to degenerate before the emergence of the insect from the egg. 



Before the revolution of the embryo the mid-gut epithelium makes its 

 appearance, covering the blind end of the stomodaeal invagination and 

 later that of the proctodaeum, the epithehal rudiments growing toward 

 each other until they meet. The dorsal closure of the mid-gut occurs 

 simultaneously with the definitive closure of the body wall. Late in 

 embryonic life, the yolk spherules, which heretofore were approximately 

 spherical, become more or less angulate: the division lines between them 

 become obliterated, the margin remaining distinct only in the periphery 

 of the yolk. Yolk-cell nuclei are now found singly or in groups adjacent 

 to the mid-gut epithelium: but since by this time the mid-gut epithelium 

 has already formed as a very thin layer, the yolk-cell nuclei do not give 

 rise to this epithelium as in Lepisma. 



The revolution of the embryo takes place as described for Pediculus, 

 the amnion and serosa secondarily fusing at the cephalic end of the 

 embryo and then rupturing to permit the eversion of the embryo. 



THYSANOPTERA 



Thrips {Thrips physapus L.) 

 The development of Thrips physapus has been briefly described by 

 Uljanin (1874). The species occurs in the blossoms of the Cichoriaceae 



