Social Insects Sociality evolved multiple times in insects Much of Earth s fauna consists of social insects They play major roles in entire ecosystems Proliferation of ants and termites associated with change from solitary to social lifestyle Social Insects Consistent trend: Numerous forms of subsocial in numerous lineages Aggregations Parental care Parental care & nesting Communal nesting Increasing division of labor and reproduction in fewer lineages Primitively eusocial in fewer lineages Advanced eusocial fewer lineages More widespread 13 orders of insects no reproductive division of labor Subsocial 1
Eusocial Three traits Some individuals in colony reproduce, others sacrifice reproduction Cooperation in tending young Overlap of generations within colonies Ants, termites, some bees and wasps Aggregations as subsocial behavior Food exploitation bark beetles Roosting Aposematism monarch butterflies ladybird beetles Chemical defenses Sawflies Parental care Aggregations as subsocial behavior Food exploitation bark beetles Roosting Aposematism monarch butterflies ladybird beetles Chemical defenses sawflies Parental care 2
What parental care do all insects provide? Post hatching attention, provision and protection of food Without nesting Tending eggs and young Care of nest Can involve feeding Blattodea, Orthoptera, Dermaptera, Hemiptera, Coleoptera and Hymenoptera Parental care Membracid nannies Parental care by ants. Ants obtain honeydew from treehoppers Nesting as social behavior Eggs laid in structure and parents tend young Types of nests New construction Pre-existing Types of care Vigilence Nutrition 3
Nesting as social behavior Solitary nesting No reproductive division of labor Examples: ground nesting crickets, earwigs, some beetles, Hymenoptera What orders? Nesting is not known in other orders. Nesting as social behavior Most nesting subsocial Hymenoptera are parasitoid wasps. Communal nesting Nests shared among individuals Parents clean, provision, defend nests Conflicts among nest mates common Halictinae, Megachilinae, Andreninae 4
Subsocial aphids & thrips Aphids Behavioral and morphological differences 1. Soldiers All first instar molt into normal feeding individuals Reproduction delayed if molt is delayed 2. Special caste of soldiers Subsocial aphids Soldier nymphs Adults Genetically identical clones by parthenogenesis Remain at 2nd instar without molting and growing. Sclerotized cuticle Sterile Attack natural enemies & competitors Perform housekeeping. Differences likely attributable to differences in gene expression between the castes POLYPHENISM Molt and grow. Soft cuticle. Contribute to reproduction. Perform neither defense nor housekeeping. Reproductive (normal) nymphs Subsocial aphids Soldiers injecting venom into predatory lacewing larva 5
Subsocial aphids & thrips Thrips Behavioral and morphological differences Dispersers & soldiers in colony in gall Soldiers defend gall against other thrips species and usually reproduce less than dispersers Subsocial thrips Dispersers are highly fecund, participate in extensive brood care, and are adept at initiating and growing new galls. Soldiers have low fecundity and are involved primarily in defense of the gall from kleptoparasites. They cannot initiate or grow galls. Quasisociality & Semisociality Communal nest- Members of same generation cooperate in brood care Quasisociality- All females can reproduce Semisociality Division of reproductive labor Workers are sisters to queens No morphological difference between queen and workers 6
Eusociality Multigenerational colonies Polyphenism: morphological differences between queen and workers Polyethism: behavioral differences Caste system Reproductives Workers Soldiers Subcastes Found only in Hymenoptera and ALL Isoptera Natural selection & Eusociality Why would some individuals NOT be involved in reproduction? Natural selection & Eusociality W.D. Hamilton gave us the tools to solve one of Darwin s greatest challenges. The evolution of altruism. 7
Eusocial Hymenoptera Why are there so many origins of Eusociality in the Hymenoptera? How are r and B maximized with respect to C? Super-relatedness in Hymenoptera What sex-determination system do all Hymenoptera have? So, what is the genetic relatedness between sisters? How could this result in a maximization of r? What benefits might there be of this extensive social behavior? Eusocial Hymenoptera Which is/are the male(s) in the above figure? Which is/are the female(s) in the above figure? Feeding quality + substance yields polyphenism in diploids, which are all. 8
Hymenoptera Numerous origins of eusociality from subsociality. Primitively eusocial Females morphologically similar Colonies usually annual Advanced eusocial Ants, some wasps, many bees Behavioral and morphological differentiation Workers different than queens Primitive eusocial Hymenoptera Numerous transitional lineages, e.g.: Polistine Wasps, a few others >1 female forms colony Colony lasts 1 yr Variable # reproductives Bumblebees, Halictinae >1 female forms colony winning female reproduces and is aggressive Sexual retardation reversed if queen dies Bumblebees Colony foundation by one or more females Pheromones used to modify worker behavior Ovarian development in late season workers Queen may be driven from nest Workers produce male offspring parthenogenetically 9
Advanced eusocial Hymenoptera Many bees and some wasps, ants Females dimorphic Specialization of workers Wasps Queen founds first brood of workers Subsequent generations include males, then reproductive females Eusocial honeybees (Apidae) Colony develops and may found new colonies during summer Nest made of wax secreted by bees Castes: queen (larger), worker (smaller), drone Old workers: hive; Young workers: field Extreme eusociality: ants (Formicidae) All ants are eusocial Workers may be polyphenic Trophogenic Feeding Predatory Seed and grain harvesters Mutualists with plants Honey dew specialists Fungus harvesters Parasites (slave-makers & usurpers) Amblyopone are specialist predators of centipedes 10
Extreme eusociality: ants (Formicidae) All ants are eusocial Workers may be polyphenic Trophogenic Feeding Predatory Seed and grain harvesters Mutualists with plants Honey dew specialists Fungus harvesters Parasites (slave-makers & usurpers) Extreme eusociality: ants (Formicidae) All ants are eusocial Workers may be polyphenic Trophogenic Feeding Predatory Seed and grain harvesters Mutualists with plants Honey dew specialists Fungus harvesters Parasites (slave-makers & usurpers) 11
Extreme eusociality: ants (Formicidae) All ants are eusocial Workers may be polyphenic Trophogenic Feeding Predatory Seed and grain harvesters Mutualists with plants Honey dew specialists Fungus harvesters Parasites (slave-makers & usurpers) Polyerges Slave-maker Formica Slave A queen Polyergus slave-raiding ant attended to by a Formica worker. Parasitic queens such as Polyergus emit pheromones that are attractive to host ant workers Eusocial Isoptera What sex determination system do termites have? Uh-oh, zup? Perhaps something about maximizing B when feeding on wood? What else happens to wood-feeding insects in their population structure (foreshadowing Normark paper)? 12
Eusocial Isoptera In what way does this caste determination structure differ from Hymenoptera? Lower termites Wood-digesting endosymbionts Loose castes, all workers immature Morphology and caste may change after molting Queen similar to workers Termites Higher termites: Termitidae No symbiotic flagellates How do they digest wood? Rigid caste system Queen differs from workers: Physogastry Termites 13
Termite mounds 14