is a characteristic of certain arthropods (mostly insects and mites), where the abdomen is greatly enlarged and membranous

The most common examples are the "queens" of certain species of eusocial insects such as termites, bees and ants, in which the abdomen swells in order to hold enlarged ovaries, thus increasing fecundity. This means that the queen has the ability to hold more and produce more eggs at one time. Physogastric queens produce an enormous number of eggs which can account for a significant amount of their body weight. In the termite species Macrotermes subhyalinus, eggs can make up a third of their body weight, and a 15-gram queen can produce up to 30 eggs per minute. The physogastric queens' egg production is supported by oocyte proteins supplied by the "queen body fat."

Various lineages of non-social insects demonstrate various degrees of physogastry, especially among females. The most dramatic and best-known are among the beetles, especially certain groups that live inside ant or termite nests, where the distended abdomen is thought to enhance integration into their hosts' colonies, though it is also seen in several herbivorous beetle lineages such as Chrysomelidae (e.g., Gastrophysa) and Meloidae (e.g., Megetra).