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CASTE BIAS Among Arthropods
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Generalizations
on Caste Determination
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Characteristics Historically castes have been recognized in insects 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
which 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). Queens are produced by overfeeding, and effect of
the workers eating most of the eggs forming the dry weather brood, 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 (S. E. Flanders, pers. commun. E.
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. |