Worker Size and Nest Defense in Solenopsis invicta (Hymenoptera: Formicidae)

Abstract

Workers of the red imported fire ant, Solenopsis invicta Buren (Hymenoptera: Formicidae), range in size from ≈0.5 to 1.5 mm in headwidth. Such size variation is thought to enable a more efficient division of labor, because some worker sizes may be better suited to performing particular tasks. Here, I focus on the task of nest defense, where efficiency and effectiveness may depend on having appropriately sized workers respond to particular types of threats. Specifically, I tested whether the size of responding workers differs between two magnitudes of nest disturbance (light, insect-like versus heavy, vertebrate-like), and I investigated how both sting length and the amount of venom workers contain, key defensive traits, vary with worker size. Workers responding to heavy, vertebrate-like nest disturbances were larger than those responding to light, insect-like disturbances. Between the light and heavy disturbances, the proportion of minor (small) workers responding dropped, whereas that of major (large) workers increased five-fold. Although the relationship between sting length and worker size is not directly proportional, the average sting length of majors is ≈0.20 mm (or 40%) greater than that of minors, with little overlap in range. The relationship between worker size and the amount of venom they contain is also positive. So, vertebrate-magnitude nest disturbances are met with greater proportions of large S. invicta workers than are insect-magnitude disturbances, and these larger workers bring with them longer stings and larger venom arsenals, which may increase their effectiveness against such relatively large, thicker-skinned threats.

Assessing ploidy levels and karyotype structure of the fire ant Solenopsis saevissima Smith, 1855 (Hymenoptera, Formicidae, Myrmicinae)

“…The queen, or sometimes queens in polygynous species, is responsible for reproduction, while the workers build the nest, defend the colony, and are responsible for obtaining and handling resources (Wilson 1998). The division of labor can also be related to the morphology of each worker, in which the sizes and ages of the workers will define which function they perform within the nest ( Haight 2010 ).…”