Morphology and physiology of larvae of the imported fire ant, Solenopsis invicta Buren

Abstract

The larvae of the imported fire ant, Solenopsis invicta Buren have 4 larval instars, as determined by observation of molting and by comparing newly molted larvae with those ready to molt to the next stage. First instar larvae are hairless. Second instar larvae have only a few small, simple hairs. Third instar larvae have moderately numerous, short hairs - either simple, branched, or bifid. Fourth instar larvae have long, straight, simple hairs on the head and anteroventral body region; all other body hairs and some posterior head hairs are strongly bifid. The mandibles of only the 4th instar larvae are sclerotized. Morphological changes correlate with changes in functions. Both eggs and larvae stick together in clumps in the colony. This process is facilitated in eggs and 1st and 2nd instar larvae by means of an adhesive coating, and in 3rd and 4th instar larvae by hooked hairs which interlock on adjacent larvae. Morphological changes also correlate with changes in behavior. Workers place solid food, of a specific size range, on the anteroventral region of 4th instar larvae which is specialized to hold food. Workers feed liquid foods, including sucrose and soybean oil, to larvae of all instars, and feed solid proteinaceous foods only to the fourth instar larvae. Gross internal anatomy of larvae of the imported fire ant is similar to that found in larvae of other ants. The muscular foregut opens into a large blind midgut which fills much of the body cavity. The fat body containing urocytes is prominent throughout life. Gonopodal discs are distinctly different in male and female 4th instar larvae. Ultrastructural and histochemical studies indicate that the labial gland tubes produce and secrete a proteinaceous substance rich in digestive enzymes. The thin cells of the labial gland reservoir have little or no secretory function, but the lumen stores the proteinaceous secretion. The labial gland enzymes include proteases and amylases, which are secreted for extraintestinal digestion of solid food. The midgut contains proteases, amylases, and lipases. Lipids appear to be ingested before digestion...