MESODERMAL DERIVATIVES 113 



of the sacs would form the splanchnic mesoderm; the outer wall, the 

 somatic mesoderm; and the cavities themselves, the definitive body 

 cavity. Escherich (1900) in his study of the muscoidean flies placed 

 special emphasis on the similarity of development between that of the 

 muscids and of the chaetognath Sagitta wherein an enterocoele is formed. 

 Wiesmann (1926), however, points out that among the Orthoptera (as 

 exemplified by Carausius), the Myriapoda, and the Onychophora, the 

 coelomic cavities form as clefts in the mesoderm and not as diverticula 

 of the archenteron. The fact that enterocoelic development does not 

 occur among the more primitive insects should prevent one from laying 

 too great a phylogenetic significance on its sporadic occurrence in the 

 muscids and other highly specialized insects. 



Coelomic cavities are lacking in the Diptera so far as known and also 

 in the lepidopteran genus Ephestia (Sehl, 1931). They are also lacking 

 in some head segments and in the last abdominal segment of other 

 insects and related arthropods. The coelomic cavity appears to be 

 lacking in the labrum of insects except in Car-ausius and Locusta among 

 the Orthoptera. A pair of preantennal coelomic cavities is present in 

 Carausius as well as in the chilopod Scolopendra, in both cases correlated 

 with the presence of rudimentary preantennal appendages. Hirschler 

 (1909) states that the beetle Donacia lacks antennal coelomic cavities, 

 the only exception recorded among those insects where sacs normally 

 occur, but Tiegs and Murray (1938) venture the opinion that the pre- 

 mandibular sac (second antennal) which Hirschler claims to have found 

 in this insect is in reality the antennal sac. Second antennal coelomic 

 cavities, either lacking or rudimentary in most insects, are present in 

 Locusta (Roonwal, 1937). They are also recorded for the chrysomelid 

 beetle Euryope by Paterson (1932) and for Calandra callosa by Wray 

 (1937). The mandibular and the first maxillary coelomic cavities are 

 either absent or rudimentary in the beetles Donacia crassipes, Calandra 

 oryzae, Hydrophilus, and some others. In insects generally, however, 

 except the Diptera, coelomic cavities occur in the gnathal and thoracic 

 as well as the abdominal segments except the last one or two. 



Among the lower Eutracheata (chilopods, Orthoptera, etc.) coelomic 

 sacs are very strongly developed, extending into the rudiments of the 

 appendages (Fig. 152, lb) and in some cases appearing more or less 

 triangular in cross section or even with large diverticula, as in Scolopendra, 

 Locusta, and Carausius. According to Korotneff (1885) in Gryllotalpa, 

 shortly before segmentation appears, the mesoderm divides along the 

 median longitudinal line into two strips. A cleft then appears in each 

 strip, thus forming two flattened tubes. Later when segmentation occurs, 

 each tube is constricted at each intersegmental region, forming the coe- 

 lomic sacs. Ayers (1884) reported a similar condition in Oecanthus. 



