294 INSECTA. 



stomodaeum and the proctodaeum and the rudiments of the limbs. 

 The fore-gut and the hind-gut appear as ectodermal invaginations, 

 the stomodaeum and the proctodaeum, in the primary cephalic region 

 and on the telson (Figs. 145 C, m, and 146 A, m and a). As a 

 rule, the stomodaeum begins to develop a little earlier than the 

 proctodaeum in the Insecta (Fig. 145 0, m). To this rule, however, 

 the Muscidae form an exception, if the observations of Voeltzkow 

 (No. 85) and Graber (No. 28) as to the early appearance of the 

 proctodaeal invagination in these forms are confirmed. 



About the time when the stomodaeal invagination appears, and 

 anterior to it in position, a forward swelling of the anterior edge 

 of the primary cephalic region is to be remarked. This is the 

 so-called procephalon (Fig. 146, vie), which represents the common 

 rudiment of the labrum and the clypeus. In many cases, this 

 rudiment first appears in the form of a small paired prominence 

 (Fig. 160, I, p. 326), which gives rise later, by fusion in the median 

 line, to an unpaired swelling which is still somewhat indented at 

 the middle. This is the case in the Coleoptera (Hydrqphilus y 

 Kowalevsky, Graber, No. 25, and Heider; in Lina, Graber, 

 No. 30 ; in Meloe, Nusbaum, No. 63 ; in Acilius, Patten, No. 67), 

 in the Lepidoptera (Tichomirofp, No. 79, and Graber, No. 30), 

 in Clialicodoma (Carriere, No. 13), and in other forms. The 

 rudiment is, on the contrary, originally single in Apis (Grassi, 

 No. 32), in Blatta (Cholodkowsky, No. 19), and in Mantis 

 (Viallanes, No. 84). The rise of the procephalon which, by many 

 authors, is called simply the labrum, from a paired rudiment has 

 repeatedly led to its being compared with a pair of pre-oral appen- 

 dages, but the grounds for such a comparison are, as we think, 

 insufficient. This view has been adopted recently by Patten 

 (No. 67), who described the procephalon simply as the first pair 

 of antennae, and also by Carriere (No. 13). The labrum of 

 the Insecta seems to us to find its homologue in the structures 

 called* by the same name in other Arthropoda (especially in the 

 Crustacea), to which the interpretation just mentioned would be 

 inapplicable. 



It should be mentioned that, in the early embryonic stages of many Insects, 

 a provisional lower lip, arising from a paired rudiment, is found just behind 

 the mouth. This is not to be confounded with the permanent lower lip of 

 the Insecta, which arises by the fusion of the second pair of maxillae. The 

 provisional lower lip was first observed by Butschli (No. 11) in Apis, and 

 called by him the inner antennae ; it was found later by Tichomiroff in the 

 Lepidoptera. Heider described it as the "lateral oral lips" in Hydrophilus > 



