CHAPTER III 

 POSTEMBRYONIC DEVELOPMENT 



M. B. CHITWOOD 



Except for size, reproductive organs and related 

 structures the majority of nemas are fully developed at 

 the time of hatching. The primitive nemas, having no 

 increase in cell number in most organs, undergo no gross 

 morphological changes. However some of the more 

 highly specialized groups undergo changes in the char- 

 acter of the labial region, stoma and esophagus, as well 

 as changes in internal structure. 



There is no true metamorphosis in nemic development 

 comparable to that occurring in insects since tissues are 

 not destroyed and rebuilt. Changes in gross body form 

 are, for the most part, of proportion rather than struc- 

 ture. Heterodera is the most striking example of modified 

 body form. In this genus the first stage larvae are typical 

 "eelworms" while the preadults are thickened and sac- 

 like. The female continues enlargement, assuming a 

 pear-shape in the adult stage (Fig. 115 N) while the 

 male returns to the previous thread-like appearance 

 (Fig. 163 N). 



Most of the developmental changes are paling enetic, 

 that is, features are derived from long evolution and 

 concerned with adult existence. However, many cenogene- 

 tic features occur which are purely larval adaptive fea- 

 tures, interpolated into development to aid the larva in 

 coping with its own separate existence. Developmental 

 changes may best be considered system by system. 



Cuticle. Ordinary postembryonic developmental changes 

 of the cuticle are limited to the thickening of the cuticle 

 and the development of such structures as caudal alae 

 in the male. Many cenogenetic features appear after 

 hatching, disappearing before or with the last moult, while 

 the majority of palingenetic modifications appear with 

 the last moult. 



The caudal alae and papillae of the male develop in 

 the fourth stage larva when the tissues of the body draw 

 away from the old cuticle and the adult structures are 

 formed beneath it (Figs. 156 S-T, AA & DD). The spines 

 of adult Hystrignathus and cuticular plaques (Fig. 23, 

 SSS p. 22) of adult Gongylonema (according to illustra- 

 tions by Alicata, 1935) appear in the fourth stage larvae 

 while the collarettes of Spiniteetus (Fig. 23, p. 22), spines 

 of Hystrichis and the large trifid lateral alae of Physo- 

 cephalus sexalatus, according to Seurat, 1913, first appear 

 at the last moult. Cuticular inflation around the head 

 (Fig. 23 RRR) of trichostrongyloids (Longistriata has- 

 salli) first appears in the pre-adult stage. Roberts (1934) 

 reports that in the third stage larvae of Ascaris hunbri- 

 coides well developed lateral "membranes" are present, 

 becoming very broad and fin-like in the fourth stage, 

 diminishing in the fifth stage. As has been previously 

 mentioned (p. 25), lateral alae are often much larger 

 and wider in the larvae than in the adult, this being 

 particularly true in the members of the Oxyuridae and 

 Thelastomatidae. It is interesting to note that such 

 wide alae are not present when the larva is pressed from 

 the egg shell. Lateral alae probably function as "wings" 

 or "immovable fins" to assist the larvae in locomotion 

 and are no longer necessary once the nema has settled 

 in a suitable place. 



The remaining cuticular modifications appear to be 

 purely cenogenetic. Wetzel (1931) described four large 

 hooks placed in dorsolateral and ventrolateral positions 

 in the fourth stage larvae of Dermatoxys veligera. These 

 hooks were lost at the subsequent moult. 



Change in size and shape of the tail is, perhaps, the 

 most common post-embryonic phenomenon. In Hystrig- 

 nathus the tail is distally bifurcate during the first three 

 larval stages (Fig. 155) while in Strongyloides it is 

 digitate only in the infective larvae. Buds appear on the 

 tail of the third stage larvae of Strongyloides before its 

 emergence from the cuticle of the second stage. Among the 

 Strongyloidea the occurrence of a long thin filiform tail 

 in the second stage larva causes the infective larva to 



Fig. 155. 



Hystrignathus? rigidus, larval female showing forked tail. After 

 Christie. 1934, Proe. Helm. Soc. Wash., v. 1 (2). 



have an even longer whip-like tail which is practically 

 diagnostic for some groups (Fig. 99); the tail of the 

 third stage larva itself usually is quite short even conoid. 

 The tail of the second stage larva of trichostrongyles is 

 shorter, conoid, attenuated with the tail of the third 

 stage even more conoid inside it (Trichostrongylus axei, 

 Fig. 158 D) or it may bear papilla-like digitations, 

 (Ornithostrongylns quadriradiatus Fig. 158 U) which are 

 subsequently lost while the pronged tail persists to the 

 adult in Ollulanus and Tricholeiperia. In the Metastrongy- 

 loidea (Figs. 156 U-V & 158 T) peculiar and characteristic 

 notching of the tail under the cuticle is evident in the 

 first or second stage becoming very well marked in the 

 third stage and disappearing with the third moult. 



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