Female, 80-200 mm. or more, width 0.5 mm., a = 200-250; V
Male, 40-50 mm. long
With characters set
out above and cuticle smooth, with criss-cross fibres below.
Body tapering in front. Head slightly offset, rounded in front. Oesophagus
comparatively short and indefinite. Immediately behind head an orange-red
coloured area is found in egg-laying females. This is the 'chromatrope'
of Cobb which is a light-sensitive region and is the organ (?) whereby
the worm responds to light during egg-laying. Tail rounded or sub-conical.
Vulva practically equatorial. Eggs globular, 54u in diameter and
50-53u from pole to pole. Very characteristic in shape, with an outer
covering showing an equatorial constriction, but with the poles produced
into a kind of stem provided with numerous fine, thread-like branches by
means of which the eggs become attached to the surface of leaves and stems,
etc. Male with arcuate spicules. Males are apparently much
less common than females but perhaps the apparent discrepancy in numbers
is due to the males remaining buried in soil and not coming to the surface
as the females do.
Bionomics. Females of Mermis nigrescens are cream
to brownish-black in colour when seen above ground during egg-laying.
The colour depends largely on the quantity of eggs stored in the uteri;
if full the worms will be dark in colour; if nearly empty they will be
cream or even white. The worms occur most abundantly in England in June
and particularly during warm showery weather they may be found moving
on the surface of the soil or on the stems and leaves of plants often 1-2
ft. above soil level. Whilst the major part of the body clings to
the substratum, the head end will be found raised up and waving to and
fro as though in search of something. The female under such conditions
is laying eggs on the surface of the foliage on which she moves.
The eggs, by reason of the special polar branching filagments (see
fig. 297b), cling to the surface on which they are laid. Further
development depends on their being eaten by some suitable insect, such
as an earwig of grasshopper, or some other phytophagous arthropod.
Within the gut of the insect the outer coat of the egg becomes rubbed off
and the larva makes its way out of the inner shell. It is provided
with a mouth spear with which it pierces the gut wall of the insect and
passes into the body-cavity. Here further development takes place.
According to Christie (1937) larvae destined to become females remain 8-10
weeks or longer in the host whereas those destined to become males remain
4-6 weeks. Such larvae, measuring from 50-160 mm. for females and
20-60 mm. for males, finally make their way out from the host (which is
generally killed in the process) into the soil. Here they remain
until the following spring when the final moult takes place and they
become fully adult in structure. Copulation takes place in the soil
and the eggs begin to mature in the females. The latter, however, remain
in the soil and egg-laying does not begin until the following spring and
summer. Thus it is only larvae which have emerged from insects and
adult worms which occur free in the soil.
(Description- Goodey, 1963)