Manual for Diagnosis of Screw-worm Fly - xcs consulting

The egg of C. bezziana is white, 1.25mm long and 0.26mm in diameter, with a cylindrical
shape, rounded at both ends but with one (anterior) end more tapered. Hatching lines
enclosing a median strip occur along the entire length of the dorsal surface of the egg,
occupying about 20-30 per cent of the width of the egg and bifurcating at the micropyle to
produce a horse-shoe shaped structure (Fig. 11). Within the median strip, there is a plastron
network which facilitates respiration in the developing embryo. The shell or chorion of the egg
is comparatively thick and hard. On squeezing with forceps, the rupturing of the chorion is
quite audible.
Figure 11: Egg mass and egg of Chrysomya bezziana.
3.3 Larvae
Agriculture, Fisheries and Forestry - Australia
There are three instars during development of C. bezziana larvae. The first two instars each
occupy one day and the third and final instar lasts three to five days. They have been
described by Kitching (1976). There are 12 visible segments: the head, three thoracic and
eight abdominal segments. The posterior spiracles of first, second and third instar larvae
each have one, two or three openings respectively to the tracheal system (Fig. 12).
First Instar (Figs. 12,13)
The first instar larva is white, 1.6mm long and 0.25mm in diameter, round in cross section,
without obvious protuberances or papillae, and with bands of spines on most of the body
segments. The spines are thornlike, black in colour and backwardly directed. The prothoracic
spiracle is a small, simple opening. There is a pair of posterior spiracles each with a single
opening but with no peritreme.
Second Instar (Figs. 12,13)
Second instar larvae are white to cream coloured, 3.5-5.5mm long and 0.5-0.75mm in
diameter. Spines in bands, thornlike, black, directed backwards, all with a single point.
Protharacic spiracles are non-functioning (occluded) with 4-5 stubby, lightly sclerotised
papillae or branches. Each of the pair of posterior spiracles is surrounded by a heavily
sclerotised peritreme, dark brown to blackish in colour, which is incomplete or nearly so
dorsally and ventrally, and has 2 slit-like spiracular openings.
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