EARLY DEVELOPMENT 45 



The gnathal appendages, representing rudiments of the mouth parts, 

 developed in the embrj^o as distinct pairs of evaginations with the corre- 

 sponding nerve gangha and coelomic sacs are well marked and distinctly 

 separated (Fig. 29). The second maxillae late in embryonic life fuse 

 along the median line to form the labium. It has been shown by Hey- 

 mons (1899), Muir and Kershaw (1911), and others that the very special- 

 ized mouth parts of the Heteroptera develop from the usual embryonic 

 outgrowths such as are formed in the embryos of insects with biting 

 mouth parts. Shinji (1919) made a similar observation regarding the 

 mouth parts of the Coccidae. The ventrolateral, thoracic, embryonic 

 evaginations which develop into legs together with the corresponding 

 nerve ganglia and coelomic sacs likewise are clearly differentiated and 

 cannot be misinterpreted. 



Attention has been called to paired evanescent, embryonic, ventral 

 evaginations of the first abdominal segment in insects by Graber (1889a), 

 Wheeler (1889a,c, 18926), Carriere (1891a), and others. Hussey (1926) 

 has given in addition to her own contribution a review of the work of 

 earlier writers, which may be summarized as follows: The pleuropodia 

 of insect embryos are paired appendages of the first abdominal segment 

 which arise from foot-like organs and which tend, as they develop, to 

 take up a position on the pleural wall of the embryo. They are serially 

 homologous with the appendages of the head, thorax, and abdomen. In 

 the orders Dermaptera, Hymenoptera, and Lepidoptera they are present 

 for a very brief period as minute, papillate, evanescent evaginations and 

 probably are as short-lived in the orders Isoptera, Strepsiptera, Embiidae, 

 Megaloptera, and Trichoptera. They develop as bulbiform evaginations 

 with a reniform or subreniform outline in the Orthoptera. They occur as 

 pyriform or digitiform or conical evaginations in the orders Mantoidea 

 and Blattoidea. In the order Coleoptera they may be evaginated, 

 flattened, bag-like, and very large; bulbiform, digitiform; or calyculate 

 with their distal ends invaginated ; or they may be quite submerged with a 

 large orifice at the surface of the body. In the order Hemiptera they 

 arise before the revolution of the embryo as ectodermal evaginations. In 

 Belostoma and Ranatra they soon sink into the body and become bowl- 

 shaped structures, greatly increasing in size to the time of hatching. 



The pleuropodia probably serve as organs of excretion or secretion 

 during embryonic life. Blunck (1914), Slifer (1937), and others have 

 found that they furnish a hatching enzyme. In Hesperodenes, where 

 they are unusually developed, Hagan (1931) considers them to be 

 important nutritive organs for which he proposes the term "pseudo- 

 placenta," which he would also apply to the nutritive structure described 

 by Heymons (1909) in Hemimerus. Hagan (1939) has also described an 

 analogous structure occurring in a viviparous roach, Diploptera dytiscoides. 



