Eusocial Insects



usocial Insects

Eusociality, an extensively studied social system, is displayed in three main insect orders: Hymenoptera-ants, bees, wasps, Isoptera -termites, and Homoptera -aphids. Eusocial insects are recognized by three main characteristics:

1. The mother, along with individuals that may or may not be directly related, conducts cooperative care of young.

2. A reproductive division of labor evolves from sterile castes which often have certain propensities or characteristics associated with helping behavior.

3. There is an overlapping of generations which allows for the older generations of offspring to help related, younger generations.

Table 1: Examples of Eusocial Insects

Specifics Example Species
Ants All ants are eusocial.
They have morphologically
distinct workers and queens.
In some ants, the workers
do not even have ovaries.
Other workers can lay male
eggs.
Fire Ants, Solenopsis invicta
Pharoahs ants, Monomorium minimum
Carpenter Ants, Camponotus
Pseudomyrmex in acacia galls
Leaf Cutter Ants, Atta
Bees Many bees are not
social at all.
Sweatbees, Lasioglossum
Bumblebees, Bombus
Honeybees, Apis mellifera
Carpenter Bees, Xylocopa
Wasps Some wasps are eusocial,
but many are not.
Paper Wasps, Polistes
Yellowjackets, Vespula
Stenogastrine wasps
Tropical Wasps, Epiponines,
Polybia
Termites All termites are social.
They have male and
female workers and,
unlike most social
insects. are diploid
rather than haplodiploid.
Often they have a king
and a queen.
Aphids and Thrips Some aphids and thripes
are are eusocial. When
they form a gall, some
soldiers will not
reproduce. This form of
eusociality tends to be
restricted to a few soldiers,
because the sterile forms
only defend and do not
care for the young.
Therefore, there is less
potential for the development
of advanced societies.


Relatedness and the Origin of Eusociality

Many eusocial insects, including ants, bees, and wasps, are haplodiploid. Therefore, each female has two alleles at a locus, while each male has only one. This leads to a different kin relatedness than that which diploid species exhibit. For example, whereas a diploid female is related to her sister only 1/2, a haplodiploid female is related to her sister 3/4. Other degrees of relatedness are charted below:

Table 2: Genetic Relatedness in Haplodiploid and Diploid Species

Mother Sister Daughter Father brother son Niece or
Nephew
Haplodiploid
female 0.5 0.75 0.5 0.5 0.25 0.5 0.375
male 1 0.5 1 0 0.5 0 0.25
Diploid
female 0.5 0.5 0.5 0.5 0.5 0.5 0.25
male 0.5 0.5 0.5 0.5 0.5 0.5 0.25


The relatedness differences in haplodiploid species lead to differences in their display of kin selected behavior as compared to diploid species. On the basis of kin selection, eusocial females would be expected to prefer to help their mothers raise their sisters, increasing their indirect fitness, rather than concentrating on increasing their direct fitness by raising their own offspring. This explains why there are sterile castes in eusocial insects; these workers may give up potential direct benefits associated with raising their own children, because indirect benefits are so beneficial. The indirect benefits preference in social wasps can be used as a basis of comparison and analysis of the balance of indirect and direct benefits in other species.

Differences in genetic relatedness among the various individuals within a eusocial colony also make social insects important in the study of parent-offspring conflict. As mentioned above, female haplodiploids benefit from helping raise their sisters which they are related to 3/4. These females, on the other hand, are related to their brothers only 1/4. Therefore, they will prefer for their mothers to have daughters. Since the mother is related to both sons and to daughters 1/2, she will have a tendency to not show preference towards daughters, leading to a conflict of interest between the mother and her daughters.



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