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CASTE
BIAS Among Arthropods
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Generalizations
on Caste Determination
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Overview Historically castes have been recognized in arthropods
for hundreds or perhaps thousands of years, but how they came about or were
produced has remained a mystery. To this day there is still a lot of
conjecture concerning the formation of castes. Vandel (1930) stated that social insects are
characterized by the existence of two kinds of females: reproductive and
sterile (= queens and workers). The workers are neutral females that feed the
larvae and perform all kinds of odd jobs around the colony. Haydak (1943)
believed that in the honeybee the production of either a worker or a queen
was due not to the change of food, but to the different amounts of essential
nutrients consumed by the queen and worker larvae. He recognized that worker
larvae decrease in weight after being sealed in their cells, while queen
larvae actually grow after the sealing. He postulated that the anatomical and
physiological differences between the worker and the queen honeybee are due
partly to hormones, which are more activated in queens because of their
superior nourishment. Worker larvae never get enough nourishment to produce
the amount of hormone necessary for maturing the ovaries. Light (1942) stated that the general problem
of castes in social insects was the nature of the mechanisms which function
in each generation of each species to cause the offspring of the same parents
to develop, in relatively constant numbers, into several different types of
individuals which possess the special morphological features and behavior
patterns characteristic of the particular castes in the particular species,
features which are correlated with the performance of species functions in
the communal life. He believed that the food received by immature queens was
different in nature to that received by immature worker bees. In ant colonies
differential care and subsequent feeding were thought to produce the
different castes. In termites, castes (involving males as well as females)
were caused by extrinsic factors (actually intrinsic as we know it). Schneirla & Brown (1952) observed that
dry weather at the start of a brood has an impact upon the queen and colony
of Eciton ants in Panama, which in some manner apparently effects the
production of a temporary inhibition to fertilization and the production of a
few queens and many males (no workers). Overfeeding, and effect of the
workers eating most of the eggs forming the dry weather brood produce queens,
so that the relatively few larvae are over-fed with booty plus eggs. The very
few diploid eggs result from fertilization by residual sperm held over in
queens' specially modified sperm duct after an all-worker brood. Flanders (1953) believed that castes are
limited in Hymenoptera probably entirely to species in which ovulation is
externally induced. In such species ovisorption (egg resorption) is an effect
of delayed ovulation. In many social species (if not most) caste is a result
of undernourishment of the embryo through an extraction of nutriment from
ripe eggs in the posterior (caudal) end of the ovariole. In species having
many ovarioles, all ovulated eggs are worker-biased. Caste may be genetically
limited to one sex in most Hymenoptera because it is based on imaginal
(ovarian) diapause. Possibly in social species all castes are female because
males are never undernourished during development. In such social Hymenoptera
extra larval nutrition of workers counteracts its caste bias. Much is
dependent on the response of the queen (female) to her environment. Partial
ovisorption seems an
adequate explanation for workers
caste determination in social Hymenoptera. It explains the deposition, by a
single female, of eggs with different contents and volume (eggs of ants,
etc.). Complete ovisorption accounts for some females becoming nongravid
without egg deposition. Partial ovisorption could result in an undernourished
embryo. It can determine the embryo's course of development, and may result
in the production of the worker caste. Wasps and bumblebees (more primitive
Hymenoptera than Apidae) show gradations of castes. Partial ovisorption also
explains the production on nonviable eggs. Highly developed castes of ants are thought
to be genetically determined, but realized only phenotypically. Factors
regulating the amount of nutrient extracted from the ripe eggs are thought to
be environmental. Involved are relative humidity affecting the ovisorption
rate, and oviposition response of the female which regulate the amount of
exposure of the eggs to the ovisorption process. Prototypes of castes are found in ants.
There are no structurally developed castes incapable of copulation in wasps
and bumblebees. There are temporary gravid and nongravid types of females in
some pteromalids (e.g., Peridesmia, Spintherus, Dibrachoides). Wilson (1953) rejected Flanders' hypothesis
of caste determination being based on ovisorption. He believed that the
preponderance of available evidence indicates that the caste of female
individuals is determined in the larval period, without regard to the
original condition of the egg. He admitted that Flanders' idea involving
ovisorption had a good chance of holding under conditions of complete
dimorphism (e.g., honeybee); but for other species he thought that
ovisorption might exercise a subsidiary influence in caste determination by
statistically affecting the chance of a larva attaining the important size
levels during its growth. In his final argument against Flanders' hypothesis,
Wilson in effect embraced the hypothesis afterall! But only after a display
of text sassyness and confusion! [see Wilson 1963, 1968]. In an elaboration on his hypothesis,
Flanders (1957) cites evidence by Bier (1954) on Formica rufa
showing that the worker derives only from ovarian eggs which have a reduced
amount of yolk. The predisposition to become a worker, however, could be
counteracted by heavy feeding during the early stages of larval development. It is thought that queens of highly
organized hymenopteran societies (e.g., army ant, honey bee) have lost the
capacity to deposit yolk-replete eggs. The occasional occurrence of a
worker-sized queen may be the result of an egg developing into reproductive
females regardless of the nutrition of the larvae. All the fertilized eggs
deposited by such a queen are predisposed, by the reduced amount of yolk, to
become workers. The larvae from such eggs, however, may become queens if they
receive materials that inhibit development into workers and stimulate
development into queens. Flanders considered the assumption false that the
worker caste in the honeybee is initiated during the larval stage. Snodgrass (1956) referred to a larval diet
containing specifically inhibitory ingredients that suppress the development
of worker characters. The larger size of a queen cell also may inhibit the
development of a worker therein. This comes about because all the food must
be consumed or pupation cannot occur. Flanders concluded that the queen in the
more primitive social species may be derived either from a fertilized,
yolk-replete egg or from an adult worker in which nutricial castration is psychologically
counteracted. He believed that the ovigenic-ovisorptive cycle which
characterizes the social Hymenoptera plays, in the honeybee, a basic role in
the economy of the species by (1) allowing greater flexibility in oviposition
so that the queen can take full advantage of the number of brood cells as
they become available, and (2) the fact that the ovigenic-ovisorptive cycle
correlates with numerous ovarioles to predispose all the eggs deposited by
the queen to become worker-biased (males if unfertilized). It appears that in the ant, caste formation
is initiated in the ripe ovarian eggs. As Whiting (1938) suggested, slight
stimuli applied at a nutritive-effective period in the ant egg could account
for the wide differences between the worker, soldier and sexual female. Generalizations
on Caste Determination (Dr. Stanley E. Flanders, personal communication to Dr. Erich
F. Legner) The worker caste
consists of female adults in imaginal
diapause. Imaginal diapause is facultative and reversible in parasitic
species and in primitive social forms. Imaginal diapause occurs only in
species in which reproduction is synovigenic, ovulation is externally
induced, and unovulated eggs are resorbed (the number of eggs resorbed may
exceed those deposited. In the higher
social Hymenoptera imaginal diapause is obligatory and is fixed by
morphological differentiation. Some workers may generate eggs if queens are
lacking, but these are able to produce worker progeny only by thelytoky. Caste formation
(imaginal diapause) is adaptive, being phenotypic in inception. A single
queen when her environment is appropriate, can repeat a production sequence
of workers, soldiers, males and queens. Colonies of ants living parasitically
on other ant colonies may never produce "workers." Imaginal
diapause is initiated prior to ovulation. Normally, all ovarian eggs are male
(haploid) and all are subject to possible resorption. Partially resorbed eggs
are deposited without impairing their development. If too much yolk has been
extracted the embryos die in all stages of development. Adult progeny
from eggs generated in one ovary of a single female may consist of both
diapause and nondiapause females. Logically this is an effect of differential
embryonic nutrition. Larval nutrition by counteracting the effects of
embryonic malnutrition can change a prospective diapause female (worker) into
a nondiapause "queen." However, the parasitic extraction of
assimilated food from a prepupal queen ant can cause it to become a
worker-like adult. Exercise 24.1--Define caste bias. Exercise 24.2--How many different castes are known among arthropods? Exercise 24.3--How may castes be determined? REFERENCES: [Additional references may be
found at MELVYL
Library ] Bellows, T. S., Jr. & T. W. Fisher, (eds) 1999. Handbook
of Biological Control: Principles and Applications. Academic Press, San Diego, CA. 1046 p. Flanders, S. E. 1953. Caste determination in the social Hymenoptera. Sci. Mon.
76(3): 142-48. Flanders, S. E. 1957. Regulation of caste in social
Hymenoptera. J. New York. Ent. Soc. 65: 97-105. Flanders, S. E. 1960. Caste in the honey bee. Insectes Sociaux 7: 7-16. Haydak, M. H. 1943. Larval food and development of castes in the honeybee. J. Econ. Ent. 36: 778-92. Light, S. F. 1942. The determination of the castes of social
insects. Quart. Rev. Biol. 17: 312-06. Light, S. F. 1943. The determination of the castes of social
insects II. Quart Rev. Biol. 18: 46-63. Schneirla, T. C. & R. Z. Brown. 1952. Sexual broods and
the production of young queens in two species of army ants.Zoologica 37: 5-32. Vandel, A. 1930. La production d' intercastes
chez la fourmi Pheidole pallidula sous l'action de parasites du
genre Mermis.Bull. Biol. France
& Belg. 64: 457-92. Wilson, E. O. 1953. On Flanders' hypothesis of caste
determination in ants. Psyche
60: 15-20. Wilson, E. O. 1963. The social biology of ants. Ann. Rev. Ent. 8: 345-68. Wilson, E. O. 1968. The ergonomic of castes in the social
insect. Amer. Nat. 102(923): 41-6. |