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TEXT-BOOK OF ENTOMOLOGY 



It is now generally believed that there are primarily eleven abdominal seg- 

 ments, while Heymons has detected twelve in the embryos of Blattids and 

 Forficula (see p. 162). In the later stages of embryonic development the num- 

 ber of abdominal segments is diminished, the 10th and llth abdominal segments 

 becoming fused. In Hydrophilus and Lina this is the case, but according to 

 Graber, in the Lepidoptera there is a fusion of the 9th and 10th abdominal seg- 

 ments, the llth remaining free. 



According to Wheeler, in Doryphora, and also in Chalicodoma (Carriere), 

 between the primary head-region and the mandibular segment is interpolated a 

 rudimentary and transitory body-segment, the premandibular segment. Ac- 

 cording to Carriere this segment corresponds to a rudimentary pair of limbs, 

 and also to a ganglion, which participates in the formation of the oesophageal 

 commissure (see p. 51). 



The procephalic lobes. The head-lobes, or procephalic lobes, ap- 

 pear at a very early period (Fig. 524, kl), before any traces of the 



FIG. 524. Three embryonic stages of a leaf-beetle (Lina) : A, unsegmented primitive streak; 



into the thoracic region. After Graber, from Korschelt and H eider. 



segments of the trunk region. Ayers has shown that in (Ecanthns 

 the primitive band, in its earliest condition and before the appear- 

 ance of the head-lobes, is a simple oval plate or almond-shaped 

 thickening near the posterior end of the egg (Fig. 525, i, 2). This 

 plate is "soon divided into two tolerably well-marked regions by 

 the enlargement of the head-end," the first indication of the head- 



