This pattern is illustrated by the adjacent figure and
consists of two phases, parasitic and pre-parasitic. The parasitic phase takes
place inside the definitive host while the pre-parasitic phase occurs either as a
freeliving phase in the external environment or inside a second host, called
an intermediate host. This basic life cycle also consists of seven stages, an egg,
four larval stages (L2, L2, L3, L4) and two adult stages comprising separate males and
females. Sometimes the sexually immature adult stages are called L5's
In most species sexual reproduction by adult nematodes is the norm and occurs within an
infected definitive host. Eggs are laid by the female and pass from this host into the
external environment. These eggs must pass through the three developmental stages (L1, L2,
and L3) before the nematode is again infective for another host.
This is an important point to emphasize - In the vast
majority of nematode life cycles the stage that passes from the definitive host is not the
same stage that is infective for another definitive host. The nematode stage (usually an
egg or L1) that passes from a definitive host must develop through to a stage
(usually the L3) that can then infect another host.
A first stage larva develops inside an egg, then hatches.
Initiation of the hatching process is controlled by several factors including temperature
and moisture levels in the external environment. Hatching occurs only when environmental
conditions are favorable for survival of hatched larvae. These conditions stimulate the
enclosed larvae to assume its own role in hatching by secreting enzymes to digest the
surrounding egg membranes, then exerting pressure against the weakened membranes to
rupture them and escape.
This newly hatched L1 feeds on bacteria and grows until
constrained by its outer skin or cuticle. At this point further growth is possible only if
the larva grows a new, more flexible, cuticle and casts off its old outer cuticle. This
process is called molting and involves two steps.
1. Synthesis of a new cuticle by the hypodermis.
At this stage the larva, with a new cuticle is completely enclosed by its old cuticle.
2. Exsheathment - a process by which the old cuticle is loosened and
ruptured followed by the larva wriggling out of the casing of the old cuticle.
Nematodes molt four times during each life cycle with a molt
occurring at the end of each larval stage. Therefore, molts separate the first and second
larval stages (L1 and L2), the second and third larval stages (L2 and L3), the third and
fourth larval stages (L3 and L4) and also the fourth larval stages and immature adults (L4
and L5). The L5 grows to the limit of its new cuticle, at the same time developing into a
sexually mature adult male or female.
This developmental cycle can be represented by a growth curve as
shown in the following figure.
An (L1) develops inside the egg, hatches
(H), grows
rapidly then molts (M1) to an L2. This second stage larva also shows a rapid spurt of growth followed by a
second molt (M2)
to a third stage larva (L3) the infective stage for many nematode species. This (L3) grows then molts(M3)
inside the host to an L4. This final larval
stage grows and undertakes a final molt (M4)
to an immature adult (L5).
These L5's pass through a final growth phase to become sexually mature adult males and
females.
