298 INSECT A. 



doptera and Hymenoptera, the limb-rudiments of the first abdominal 

 segment are, in some cases, less developed than those of the other 

 segments, and in no case do they attain a greater development. 



In the Orthoptera and Coleoptera, the limb-rudiments of the first abdominal 

 segment show, as is often the case with vestigial organs, considerable variability 

 in their later development. They are most leg-like in Mantis, according to 

 Graber, and in this genus, as well as in some other forms, they even show 

 signs of segmentation, the finger-like process appearing divided into two by 

 a transverse constriction. The limb-rudiments of this segment in Melolontha 

 attain an altogether excessive development (Fig. 147 B, a 1 , Graber), being 

 transformed into large vascular sacs, the walls of which seem to be composed 

 of massive coarsely-granular elements. In many other cases a glandular sig- 

 nificance is suggested for these appendages, the walls at their distal parts being 

 formed of very large coarsely-granular glandular cells which are often pigmented. 

 In such cases, the appendages are mushroom-shaped (Gryllotalpa, Hydrophilus) 

 or, when the distal glandular surface sinks in, they assume the form of stalked 

 cups (Mcloc, Nusbaum). Finally, they may be represented by a sac sunk 

 below the surface of the body (Tenebrio, Carriere), or a similarly-shaped 

 solid structure (Cicada and Nepa, Wheeler). The different shapes assumed 

 by this structure are connected by means of many transition forms. The 

 secretion which has been observed may be gelatinous (Mcloc, Nusbaum, and 

 Cicada, Wheeler) or filamentous {Nepa, Wheeler). The physiological sig- 

 nificance of these organs still seems very obscure, in spite of the observations 

 which have been published ; they have been claimed as embryonic respiratory 

 organs (gills) or glands. It should be pointed out that the character of the 

 cells here regarded as glandular agrees closely witli that of the elements of 

 the dorsal organ (invaginated serosa) before the latter begins to disintegrate. 



The appendages we have just been discussing invariably degenerate completely 

 before the larva hatches. The same is, as a rule, the case with the appendages 

 of the posterior abdominal segments, which are usually considerably smaller. 

 It is possible that when the latter disappear they take a certain part in the 

 formation of the lateral parts of the ventral plate, as was conjectured by Haase 

 (No. 153) when reviewing the condition of Machilis and Blatta, and as was more 

 recently rendered probable by Graber (No. 30) for Melolontha. 



With regard to the development of the abdominal extremities (pedes spurii 

 or prolegs) of the caterpillars of the Lepidoptera and the caterpillar-like larvae 

 of the Tenth redinidae, it appears from the researches of Kowalevsky (Sphinx), 

 Tichomiroff (Bombyx), and Graber (No. 30, Bombyx and Hylotoma) that 

 limb-rudiments first form on all or most of the abdominal segments, but 

 that they very soon disappear on those segments which, in the larva, have no 

 limbs, while on the other segments they are transformed into the functional 

 prolegs. To this view the observations of GOOSSENS and Knatz, according to 

 which single pairs of these prolegs first develop during larval life, are apparently 

 unfavourable. We should here have to suppose, as Graber (No. 30) also has 

 pointed out, an embryonic rudiment remaining for a considerable time in a 

 latent condition. On the whole, the embryological data seem to support the 

 view of Balfour, which Cholodkowsky has recently adopted, and to which 

 Graber (No. 30) is inclined, that the abdominal appendages of the caterpillars 

 of the Lepidoptera and Hymenoptera are to be regarded as true limbs. We 

 have already had several examples iu the Crustacea of the disappearance and 



