Growth and development of the phantom midge Chaoborus americanus : an assessment of developmental polymorphism
Traina, Joseph A.
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Fourth instar larvae of the phantom midge Chaoborus americanus have been described by W. E. Bradshaw in 1973 as having a genetically-based developmental polymorphism. My experiments with overwintering larvae from Tender Bog Lake corroborated that large-yellow fourth instar larvae (LYL) pupated earlier than small-pale fourth instar larvae (SPL), resulting in an extended bimodal pupation pattern. SPL pupal weights, however, were equal to or greater than LYL pupal weights, indicating that SPL were not small morphs. Census data further indicated that SPL and LYL from Tender Bog Lake are first- and second-year fourth instar larvae (respectively) from a population with a two- year generation time. Hence, the differences between SPL and LYL are ontogenetic (i.e., SPL are simply younger and developmental1y less mature than LYL). Growth and development studies demonstrated that this two-year life cycle is not the result of an obligate diapause. In contrast to Tender Bog, the pupation pattern of overwintering fourth instar larvae from Redfield Pond was unimodal. SPL pupal weights were slightly less than those of LYL in the laboratory, but greater than those of LYL in the field. Again, this indicated that SPL were not small morphs. Census data from Redfield Pond further indicated that SPL and LYL are young and old overwintering fourth instar larvae (respectively) from the same generation. Thus, the differences between SPL and LYL are again ontogenetic, and the pupation pattern produced by larvae from a single generation is not bimodal. When Tender Bog and Redfield Pond larvae were reared in the absence of food, post-diapause pupation occurred only in LYL. Consequently, food appears to be a requirement for the post-diapause pupation of small larvae only. Growth and development studies on Tender Bog larvae also demonstrated that under high food conditions larvae exhibit shorter developmental periods, faster rates of growth, and attain larger pupal sizes than larvae reared under low food conditions. Growth rates were determined in these studies by periodically measuring the tracheal front air bladder lengths of individual larvae. Air bladders are used by larvae to maintain buoyancy, and serve as excellent indicators of larval dry weight. Lastly, considerable variation in the sizes of pupae existed both within and between experiments. This variation was attributed to differences in the duration of the dormancy period, temperature, food availability, and time spent feeding.