extensively in classifying insect groups, such as 

 Diptera and Hymenoptera. It is the intention of 

 this taxonomic study to provide as much informa- 

 tion as possible so that one can examine the sys- 

 tematic value of both larval and adult characters 

 of Nematoscelis. 



MATERIAL AND METHODS 



The material used in this study consisted of 286 

 Isaacs- Kidd Mid-water Trawl (10 feet) collections 

 and 1,950 plankton samples, including those 

 collected during the International Indian Ocean 

 Expedition (1960-65). These materials came from 

 different geographical regions of the Atlantic, 

 Indian, and Pacific oceans, mostly between lat. 

 40°N and 40°S. They are deposited at the Scripps 

 Institution of Oceanography, La JoUa, Calif. All 

 the trawl collections were not quantitative for es- 

 timating species abundance. A paper on the dis- 

 tribution and abundance of Nematoscelis based on 

 plankton samples was already published 

 (Gopalakrishnan 1974). Measurements were taken 

 on 55 adult morphological characters for examin- 

 ing relative degree of differences. Ten males and 

 ten females of each species were selected for these 

 measurements. Statistical significance was deter- 

 mined on the basis of nonoverlapping confidence 

 levels (95%) of the means. For making drawings, 

 10-20 individuals of each species were treated in 

 heated 10% aqueous KOH to remove nonchitinous 

 tissues. They were then stained in 1% aqueous 

 Chlorazol Black E. Materials treated in KOH solu- 

 tion could be kept in 60-80% glycerol without 

 shrinkage. Drawings were made with the aid of a 

 camera lucida fitted to monocular and binocular 

 microscopes. 



The larval key was prepared on the basis of fur- 

 cilia characters only. However, a few comments 

 are made on characters of calyptopis and juvenile 

 stages thought to have some diagnostic value. The 

 adult key was prepared based on features of the 

 first thoracic leg (maxilliped), eyes, antenna, and 

 the carapace. Diagnostic features of the petasma 

 are also included in the present key. Many 

 characters are, therefore, used in the present key 

 to facilitate its use on juveniles and adults of both 

 sexes. Most of the commonly used adult characters 

 are illustrated in Figure la. The terminology used 

 here is the same that has been followed by most 

 other workers. Larval terminology is defined and 

 described in Gopalakrishnan (1973). 



FISHERY BULLETIN: VOL. 73, NO. 4 



RESULTS 

 Larval Development 



Between hatching and sexual maturity all 

 species of Nematoscelis pass through four 

 developmental phases: metanauplius, calyptopis, 

 furcilia, and juvenile. A modified version of the 

 nomenclature of larval development of 

 euphausiids is given in Gopalakrishnan (1973). The 

 metanauplius phase consists of one developmental 

 stage, calyptopis of three (Cj, C2, and C^ and fur- 

 cilia of three (F^, Fg, and F3). The strong 

 differentiation of mouth parts and other thoracic 

 legs shows similarities among larvae of all species 

 of this genus. The development of larvae follows 

 either of two pathways: A'', difficilis and A^". 

 megalops follow one pathway and the other five 

 species follow the other. During the third furcilia 

 stage the second thoracic leg develops spines on 

 both dactylus and propodus in N. difficilis and N. 

 megalops, whereas in the rest of the species spines 

 develop only on the dactylus. In all species of 

 Nematoscelis this leg becomes the longest of all 

 thoracic appendages. 



Other developmental differences between the 

 two species groups are as follows: during the 

 juvenile phase, the maxillules of A^. difficilis and 

 A'', megalops develop pseudexopods from the 

 posterior face of the coxa as the four-setose larval 

 exopods disappear; but in the remaining species, 

 at about the same stage, the larval exopod disap- 

 pears without the development of a pseudexopod. 

 The lacina externa (lobes of basis) of the maxilla is 

 trilobed in larvae of all species, but becomes 

 bilobed in adults of A'', difficilis and A^'. megalops 

 and single lobed in adults of the remaining species. 



The differences in the sequential development 

 of pleopods and telson spines (terminal) are sum- 

 marized in Figure 2. These features are consistent 

 and appear to be characteristic of each subgroup. 

 The terminal spines of the telson in species of both 

 subgroups show differences not only in their 

 sequential reduction but also in their external 

 morphology (Figure 3A). This structural 

 difference can be seen even during calyptopis 

 stages of all species of this genus. This is a diag- 

 nostic feature and may have significance in un- 

 derstanding the evolution of the larvae. The dorsal 

 keel and rostrum of the carapace also appear to be 

 of important diagnostic value for furcilia (Figure 

 3B) 



798 



