FISHERY BULLETIN; VOL. 78, NO. 3 



insects from being wetted by rain, spray, waves, 

 and accidental submergence (Cheng 1973b; An- 

 dersen 1977); 3) a highly UV-absorbent cuticle, 

 presumably to prevent chromosomal damage 

 (Cheng et al. 1978); and 4) an ability to store 

 relatively large amounts of food as triglycerides 

 which their brackish-water and freshwater rela- 

 tives are not known to possess (Lee and Cheng 

 1974). To date, attempts to rear pelagic Halobates 

 in the laboratory have failed. Our knowledge of 

 these insects is thus based upon analyses of pre- 

 served samples and on short-term observations or 

 experiments carried out at sea. The present data 

 are from an extensive set of neuston samples taken 

 during the EASTROPAC investigations, which al- 

 lows us to examine some aspects of species dis- 

 tribution and cooccurrence in relatively fine spa- 

 tial detail. 



METHODS 



Present samples were collected during the 

 EASTROPAC project, which surveyed the eastern 

 Pacific between lat. 20° N and 20° S from the west 

 coast of the American continents to about long. 

 120° W. There were seven cruises between January 

 1967 and April 1968, each of about 2-mo duration. 

 Figure 1 presents areas surveyed for most of these 

 cruises; some cruise tracks were complex, and 

 areas surveyed and cruise length differed from 

 cruise to cruise: details of cruise tracks are avail- 

 able (Fishery-Oceanography Center,^ and figures 

 10-70 TC in Love 1972 [EASTROPAC Atlas, Vol. 

 1]). The results of some of the biological, chemical, 

 physical, and meteorological measurements have 

 been published in several EASTROPAC Atlases 

 (Love 1970-75) and elsewhere (Ahlstrom 1971, 

 1972; Tsuchiya 1974). 



The neuston nets used to collect our samples 

 filter only the top few centimeters of water, but 

 may occasionally skip out of the water ("porpois- 

 ing"; see Cheng 1975a). Moreover, to some extent, 

 Halobates is able to detect and avoid such a net 

 both visually (Cheng 1973c; Cheng and Enright 

 1973) and by receiving tactile warnings of its ap- 

 proach (Wilcox 1972). Consequently, the samples 

 yield at best only semiquantitative data on Halo- 

 bates and other pleustonic organisms (Cheng 

 1975a). 



All the samples were replicates; a 505 (xm mesh 

 net with a circular mouth 1 m in diameter was 

 towed half submerged at about 3 kn (= 1.5 m/s) for 

 20 min. Optimally, such a tow sweeps a path 1 m 

 wide and 1,800 m long. Abundance of Halobates 

 spp. is treated as number of individuals caught per 

 standard tow; such a tow covers an area of about 

 1 ,800 m^. However, our use of 1 ,800 m^ as the "area 

 per tow" is conservative, because both porpoising 

 and variable depth of submergence will decrease 

 the actual area covered. Possible avoidance by 

 Halobates makes our abundance estimates even 

 more conservative. Samples were preserved in 

 70% ethanol. Of 1,649 surface samples, 498 con- 

 tained at least one Halobates individual. A total of 

 3,236 individuals were identified to species 

 (Cheng 1975b). For each sample, we recorded the 

 number of adults and nymphs; nymphs were iden- 

 tified to developmental stage and final instar 

 nymphs and adults were sexed. Detailed informa- 

 tion on the cruise series, the total number of sur- 

 face tows made during each cruise, and the 

 number of positive tows (i.e., containing Halo- 

 bates) are presented in Table 1. 



Table l. — EASTROPAC cruise series, number of surface tows 

 made, and number of tows containing //a/o6a<es spp. (1967-68). 



^Fishery-Oceanography Center. 1966-69. EASTROPAC In- 

 formation Paper no. 1-11 (available from National Marine 

 Fisheries Center, PO. Box 271, La Jolla, CA 92038). 



' First digit of cruise number denotes the series, except for Oceanographer 

 which IS a "ship of opportunity" used in series 50. 



RESULTS AND DISCUSSION 

 Species Distributions 



Worldwide distributions of oceanic Halobates 

 spp. have been presented by Herring (1961), 

 Savilov (1967), and Cheng (1973a, 1974). Updated 

 worldwide distributions of the four oceanic species 

 which occur in the EASTROPAC area are shown in 

 Figure 2. The general distribution of Halobates 

 spp. resembles known large-scale plankton dis- 



580 



