Determination of larval instars in Chironomus sancticaroli (Diptera: Chironomidae) using novel head capsule structures

Abstract

Determining the age composition of a population is important when conducting ecological, taxonomic and environmental assessments. Morphometric measurements of the head capsule, especially the length and width, are widely used in the identification of insect instars, but alternative ways for determining the age of insects can expand the options for the analysis of field and laboratory populations. This study evaluated the morphometry of the antennae, mandibles, mentum and ventromental plates to discriminate among the four larval instars of Chironomus sancticaroli Strixino & Strixino, 1981. The larvae were reared in the laboratory under constant temperature (25°C) and photoperiod of 12/12 hours for 19 days, with a supply of food. Fifteen larvae were removed randomly every day, cleared in KOH, and slide-mounted, and had the antennae, mandibles, mentum and ventromental plates measured. The dimensions of the four structures studied allowed us to statistically distinguish each of the four larval instars. The data fit an exponential equation according to the Brooks-Dyar rule, which allows an estimate of the larval instar of specimens collected in the field, even when development takes place under different conditions. The duration of each instar was also obtained from our data, and showed an overlap of instars during development.

Larval development and growth ratios of Odonata of the Azores

“…In this study, we used head width to detect discrete discontinuities, as in other studies (e.g. Daly 1985;Ecole et al 2008;Silva et al 2008;Delbac et al 2010;Richardi et al 2013;Cazado et al 2014). We believe that this method allowed us to accurately estimate the number of instars of each study species.…”

“…The first was the standardised and widely used method developed by Dyar (1890), based on the frequency distribution of head width (see for example Daly 1985;Ecole et al 2008;Silva et al 2008;Delbac et al 2010;Richardi et al 2013;Cazado et al 2014). In this method, we constructed a histogram of frequencies in which head width (x-axis) was plotted versus the frequencies of each measure (y-axis).…”

“…Fleck et al 2006). Morphometry is a widely used tool in the determination of larval instars (Daly 1985;Richardi et al 2013). Given that insect larvae present discontinuous growth (i.e.…”

“…One of the most commonly used variables to determine the number of instars is head width (e.g. Daly 1985;Ecole et al 2008;Silva et al 2008;Delbac et al 2010;Richardi et al 2013;Cazado et al 2014). Head width was first used to calculate the number of instars more than a century ago, and thereafter it has been established as one of the most successful methods to determinate the number of larval instars in field samples (Dyar 1890).…”

Determination of Instars of Bactrocera dorsalis (Diptera: Tephritidae)

“…Several reports on instar determination for Diptera species are available (Petitt 1990;Richardi et al 2013;Cao et al 2014;Cen et al 2015;Deng et al 2015). Measurement of either mouth hooks or the pharyngeal sclerite showed clear separation between adjacent instars of Liriomyza sativae Blanchard (Diptera: Agromyzidae) (Petitt 1990).…”