Head Size

Polistes bellicosus

Colony 114 Colony 64 Colony 155

Polistes carolinus

Colony 14 Colony 35 Colony 39

The Importance of Size

Though many factors, including age and experience, may control dominance in a colony, size is one key factor in predicting who will win fights among social insects and among other species. Experience and increased size give an individual a distinct advantage in fights over food, nesting sites, territories, and mates. These advantages can lead to a higher degree of both indirect and direct benefits.

Measuring Size
Many factors must be taken into account when measuring the relative size of individuals. In some cases, it is important to measure a specific part of the individual that directly benefits them in inner species competition. One instance of this would be if the species had horns or pinchers to use while fighting. In instances where there are no specific body parts designed for battle, it may be more beneficial to look at the general measure of weight. However, weight can fluctuat e with intake of food and water. Another general size measurement that avoids this problem is the measurement of the size of a specific body part.

In insects, because the exoskeleton is composed of multiple rigid structures, it is important to measure a body part that does not vary as the insect 'telescopes' or lengthens and contracts its body length. Head width is such a parameter. The size of the insect head is proportional to other body parts, such as wing and leg segment length, but does not constantly vary with food intake or telescoping. Once the wasp has reached full development, the wasp will no longer grow, and h ead size will remain constant.

Causes of Variation in Size of Adult Wasps
Though the head size of an individual wasp is constant once the wasp reaches adulthood, a number of factors cause variance in head size between individuals. Differing amounts of larval nutrition cause the majority of this varia tion. Because wasp larvae are helpless grubs, they are dependent on the food brought to them by adult wasps. The amount of food that the larvae receive is a factor of:

The number of adults feeding them

A greater number of adults will result in more food for each larva.

The abundance of food in the environment

Many environmental factors will affect the abundance of caterpillar (Lepidoptera larvae), the main food source for the wasp larvae. For example, if it is a wet year in which plants have grown well, there will likely be more caterpillars.

How long they remain in the larval stage

Immatures may maintain a larger adult size by remaining in the larval stage for a longer period of time. The larval stage can last from eleven to thirty days. Once a larva pupates, spinning a silken cocoon around itself and then molting into a pupa, the lengths of the growth stages become more uniform.

When they emerge

While, in Polistes bellicosus, there is no morphological caste differentiation between females emerging in the spring and in the fall, there is a size variation among wasps emerging in the spring and in the summer months. The first females to e merge in the spring are often smaller than those females who emerge later in the summer. Reasons for this phenomenon include the low abundance of caterpillar and the lower number of foraging adults who can feed the larvae in the early spring.

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© Joan E. Strassmann, reproduction by permission only
updated 13 September 1996, strassm@rice.edu