Introduction - The Social Insects; Insect Phylogeny & Evolution

True sociality or "eusociality" is defined by (from Wilson 1971) (3)

1. Cooperative brood care 

   • Members of the group help take care of the young together, not just their own offspring 

   • e.g. worker bees feed and protect the queen's larvae 

2. Reproductive division of labour – "more or less sterile individuals working on behalf of fecund individuals" 

   • Only certain individuals reproduce, while others don’t reproduce but work to support the group (gathering food, defending the colony, caring for young, etc.) 

   • e.g. the queen bee lays eggs, while workers are sterile and do all the other jobs 

3. "Overlap of at least two generations in life stages of contributing to colony labor" 

   • Parents and offspring live together at the same time, and the older generation helps raise the next one.

   • e.g. adult ants live alongside and help raise their younger siblings 

• All three conditions must be true for a species to be considered eusocial. 

Haplodiploid sex determination

A reproductive system where:

• males develop from unfertilized eggs and are haploid

• females develop from fertilized eggs and are diploid

This system promotes genetic diversity and influences social behavior in certain insects.

• e.g. Hymenoptera (ants, bees, and wasps)

Diplodiploid sex determination

A reproductive system where both males and females develop from fertilized eggs

• e.g. Isoptera (termites), Humans

Holometabolous development

Complete metamorphosis (four life stages)

1. Egg

2. Larva

3. Pupa

4. Adult

Hemimetabolous development

Incomplete metamorphosis (three life stages)

1. Egg

2. Nymph 

3. Adult

Control of metamorphosis (3)

1. Brain hormone 

   • Its job is to tell the prothoracic glands to release another hormone called ecdysone 

     • promotes growth + induces molting 

2. Ecdysone (Molting Hormone) 

   • Made by the prothoracic glands. 

   • Its job is to make the insect grow and shed its old skin (molting) 

3. Juvenile Hormone (J.H.) 

   • secreted by the corpora allata; they stop secreting JH in the last preadult instar; its 

   • absence leads to metamorphosis 

   • Its job is to keep the insect in its young stage (larva or nymph). 

     • promotes larval or nymphal development and prevents molting 

Development & Imaginal discs 

A sac-like epithelial structure found inside the larva of insects that undergo metamorphosis

• Once the larva turns into a pupa, almost all the larval tissues degenerate and the imaginal discs turn into the external structures of the head, thorax, limbs and genitalia.

• The rest of the adult external structures (abdomen) originate from nests of histoblast cells that are also formed in the larva.