44 EMBRYOLOGY OF INSECTS AND MYRIAPODS 



swellings. In some insects it arises as a pair of faint swellings, but later 

 the bifid condition is lost. In view of the paired origin in certain cases 

 and the occurrence of an independent pair of coelomic cavities lying in 

 the labrum in Carausius morosus and Locusta migratoria, some writers 

 are inclined to regard the labrum as homologous mth the gnathal 

 appendages. 



Recently Snodgrass (1935, 1938) has expressed the view that the 

 antennae of insects are not true segmental limbs but are to be regarded 

 as organs analogous to the prostomial tentacles of the annelid worm, 

 since neither in their segmentation nor in their musculature do they 

 resemble the limbs of the postoral somites, and their nerve centers are 

 preoral in position. The second antennae, which are rudimentary in 

 insects but functional in the Crustacea, without doubt represent the first 

 pair of appendages. Morphologically they are postoral, since they are 

 innervated from the postoral tritocerebral lobes of the brain. Rudiments 

 of the second antennae have been recorded in adult Campodea, Machilis, 

 Dissosteira, and Geophilus, and embryonic vestiges of them occur in the 

 representatives of several orders where they are usually referred to as 

 "second antennal," "intercalary," or " premandibular appendages." 

 Well-developed articulated appendages provided with muscles, desig- 

 nated as "premandibles," are found in the larvae of some Chironomidae, 

 Chaoborinae, Bibionidae, and other nemocerous Diptera and, although 

 they have not been studied in the embryo, may represent appendages of 

 the intercalary segment. As Wheeler, Claypole, and Folsom (1900) 

 have already pointed out, there are anlagen of premandibular appendages 

 on the intercalary segment in Anurida maritima. Folsom furthermore 

 demonstrated the presence of an independent nerve ganglion in this 

 segment. 



In the gnathal region Folsom maintained that a nerve ganglion exists 

 between the mandibular and the maxillary ganglion together with a 

 corresponding head segment and a pair of appendages which he termed 

 "superlinguae." Later authors (Philiptschenko et al.) for the most part 

 disagree with Folsom regarding the presence of an additional head 

 segment, believing Folsom in error regarding the corresponding nerve 

 ganglion and considering that the superlinguae are solel}^ lateral processes 

 of the hypopharynx. The hypopharynx itself represents the fused 

 sternites of the mandibular and first maxillary segments in the roach 

 (Riley, 1904) or of the sternites of the mandibular and both maxillary 

 segments in Locusta (Roonwal, 1937). Such origin of the hypopharynx 

 also occurs in Forficula and several Hemiptera (Heymons, 1895a, 1899). 

 In other insects it seems to be associated with the intercalary region, and 

 Hirschler suggests (1924) that the segment also shares in the formation 

 of the hypopharynx. 



