Variation of ommatidia number as a function of worker size ...

Ins. Soc. 39:233-236 (1992)

1015 1621/92/020233-04 $1.50+0.20/0 . 9 1992 Birkh/iuser Verlag, Basel

Short communication

Variation of ommatidia number as a function of worker size in Camponotuspennsylvanicus (DeGeer) (Hymenoptera: Formieidae) J. H. Klotz 1, B. L. Reid 1 and W. C. Gordon 2 1 Center for Urban and Industrial Pest Management, Departement of Entomology, Purdue University, West Lafayette, IN47907-1158, USA 2 LSU Medical School, Department of Ophthalmology, 2020 Gravier St. Suite B, New Orleans, LA 70112, USA Key words: Camponotus pennsyIvanicus, c o m p o u n d eye, ommatidia number, Polymorphism, caste development.

Summary The relation of worker size to ommatidia number was examined in the polymorphic ant Camponotus pennsylvanicus (DeGeer). Linear regression described this relationship as: Y = 260.9 + 113.6X; where Y is ommatidia number and X is head width. A log-log regression described this relationship as: Y = 323.5 + 286.9"1ogX (r 2 = 0.98). This analysis indicated an allometric relation of ommatidia number to head width, where ommatidia numbers increase at a slower rate than head width. This relationship is discussed in terms ofethotypes associated with worker morphotypes, and the possible mechanisms regulating polymorphic development.

Introduction The frequency distribution o f worker size in Camponotus pennsylvanicus (DeGeer) is bimodal, and follows a simple m o n o p h a s i c allometry (Fowler, 1986) with a clear division of labor a m o n g various-sized workers (Fowler, 1985). In Formica rufa, a positive correlation exists between the size of the corpora pedunculata, the overall head size, the size of the c o m p o u n d eye, and foraging efficiency (Bernstein and Bernstein, 1969). In Formica integroides, a continuous, positive, curvilinear relationship exists between head size, ommatidial lens diameter and ommatidia number (Bernstein and Finn, 1971). Our study with C. pennsylvanicus defines the relation of individual ant size and the constituency of the c o m p o u n d eye; this genus has not previously been studied in this regard. Our analyses establish a m o r p h o m e t r i c

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description of this size gradient, demonstrate a continual gradation in eye size among workers, and reflect the developmental processes underlying caste development in polymorphic ants.

Materials and methods

Winged reproductives and worker C. pennsylvanicus were collected from a single nest, brought to the laboratory, and anesthetized with gaseous CO2. Head width, measured as the widest point across the occiput, was obtained with a calibrated filar micrometer in the eyepiece of a Zeiss compound microscope. Corneal replicates were produced according to the method of Ribi (1978) and mounted on glass microscope slides and, with the aid of a camera lucida, the number of ommatidia were counted. The distribution of worker head widths and ommatidia counts was assessed by the Shapiro-Wilks W test. Relation of worker head width to ommatidia counts was defined by linear regression and by a log-log regression of the number of ommatidia (an approximate area measure) versus the square of head width (an approximate area measure). Regressions were evaluated for goodness-of-fit and whether slopes differed significantly from 0.0.

Results

Ommatidial counts of the left eye in ten workers ranged from 375 to 658 facets over a smooth and continuous range of head sizes from 1.2 to 3.5 mm in width. Fowler's (1985) data, from several thousand workers, showed a head size spectrum in C. pennsylvanicus workers of approx. 0.8 to > 3.2 ram. Sample data on head capsule width (W = 0.94, p = 0.49) and ommatidia count (W = 0.95, p = 0.64) were distributed normally, reflecting the continuous size polymorphism in C. pennsylvanicus adults. Fowler (1986), in regressing head length on width, also demonstrated an absence of discontinuities in worker size distributions. Figure i presents the relationship between head width and ommatidia number for C. pennsylvanicus workers. The upper graph (Fig. la) describes a significant (r 2 = 0.96), positive (t = 13.9), linear correlation between the number of ommatidia in the compound eye and worker size (taken as head width). In the lower graph (Fig. 1 b), where the linear head width was squared to approximate an area measure, the log-log plot more clearly shows the allometric relationship, where the number of ommatidia increases at a lower rate than worker head width, such that smaller workers have proportionally more ommatidia. Ommatidia numbers in winged males and females were considerably higher than in workers with equivalent head sizes (Fig. la). Males had head widths of 1.67 and 1.70 mm and 975 and 1096 facets, respectively, and females head widths of 3.35 and 3.44 mm 1025 and 1020 facets, respectively. Based on the linear regression described above, workers with head widths comparable to male or female reproductives would be expected to have about only 450 or 670 facets, respectively.

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Figure 1. G r a p h 1 a shows the relation of head width to ommatidia number in the c o m p o u n d eye. Various sizes of worker ants are compared with typical males and queens. G r a p h 1 b shows the log-log plot of head width squared to ommatidia number

Discussion

Correlations between head size and ommatidia number have been reported previously for Cataglyphis bicolor (Menzel and Wehner, 1970) and F. integroides (Bernstein and Finn, 1971), however the association between ommatidia number and ethotype was established only for C. bicolor (Menzel and Wehner, 1970). Larger workers with up to 1200 ommatidia were hunters, while smaller workers with 600 ommatidia worked as diggers in the nest. In behavioral tests, Wehner and Menzel (1969) had also demonstrated differences between those two size classes in visual orientation, where larger workers oriented more efficiently than small workers. Differences in visual orientation may exist among C. pennsylvanicus workers, although there is a narrower range in ommatidia numbers compared to C. bicolor. Smaller workers, which are usually aphid guardians, collect honeydew and pass it to larger workers, who then transport it back to the nest (Fowler, 1985). Reproductives, who must leave the nest at swarming to conduct mating flights, have a much larger number of ommatidia. We hypothesize that visual and/or behavioral variations in C. pennsylvanicus may be associated with the variable number of ommatidia represented in Fig. 1. Development controls must be exerted to produce individuals of the size gradients noted within individual nests. One likely mechanism involves a continuous developmental gradient, operating in response to external factor(s) and endocrine control. Larval nutrition, whether modulated by attendant workers, temperature and/or photoperiod, is believed to be an important factor in C. pennsylvanicus development (Fowler, 1984; 1986). In the nurseries of ant nests, the workers tending the larvae could, by altering or stopping feeding at specific intervals, determine the size of an individual when it pupates, and hence the size of the adult worker. Wheeler (1990) has shown that the monophasic allometry characteristic of Solenopsis invicta workers may arise, in part, as a consequence of hormonal regulation, and suggested that this mechanism may apply to other ant species. If the worker size sequence in C. pennsylvanicus (Fig. 1) also represents a behavioral sequence, it follows that the colony, through the actions of workers tending the larvae, could manipulate larval development by regulating their nutritional state. This would produce a continual gradation of adult worker morphotypes, with consequential gradation in ommatidia numbers, so as to give rise to the observed variations in worker ethotype.

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Acknowledgements Portions of this work were conducted when JHK and WCG were at the University of Kansas. We thank R. Jander (Univ. of Kansas) and J. Doucet (LSU/MS) for commenting on the manuscript. This is Journal Paper No. 13 091 of the Purdue University Agricultural Experiment Station, West Lafayette, Indiana.

References Bernstein, S. and R.A. Bernstein, 1969. Relationships between foraging efficiency and the size of the head and component brain and sensory structures in the red wood ant. Brain Res. I6:85-104. Bernstein, S. and C. Finn, 1971. Ant compound eye: size-related ommatidium differences within a single wood ant nest. Experientia 27: 708-710. Fowler, H. G., 1984. Colony-level regulation of forager caste ratios in response to caste perturbations in the carpenter ant, Camponotus pennsylvanicus (DeGeer) (Hymenoptera: Formicidae). Ins. Soc. 31:461-472. Fowler, H.G., 1985. Alloethism in the carpenter ant, Camponotus pennsylvanicus (Hymenoptera: Formicidae). Entomol. Gener. 11:69-76. Fowler, H.G., 1986. Polymorphism and colony ontogeny in North American carpenter ants (Hymenoptera: Formicidae: Camponotus pennsylvanicus and Camponotus ferrugineus). ZooL ,lb. Physiol. 90 : 297-316. Menzel, R. and R. Wehner, 1970. Augenstrukturen bei verschiedengroBen Arbeiterinnen von Cataglyphis bicolor Fabr. (Formicidae, Hymenoptera). Z. vergl. Physiol. 68:446-449. Ribi, W.A., 1978. A unique hymenopteran compound eye. The retina fine structure of the digger wasp Sphex cognatus Smith (Hymenoptera, Sphecidae). Zool. Jb. Anat. 100: 299-342. Wehner, R. and R. Menzel, 1969. Homing in the ant Cataglyphis bicolor. Science 164:192-194. Wheeler, D.E., 1990. The developmental basis of worker polymorphism in fire ants. J. Insect Physiol. 36: 315-322. Received 10 July 1991; revised 18 December 1991; accepted 4 March 1992.