Agriculture Reference
In-Depth Information
Many insects reproduce sexually; in others,
males may be extremely rare or are unknown
and reproduction without a sexual phase (i.e.
parthenogenesis) is normal. Members of some
groups (e.g. many aphids) reproduce partheno-
genetically and viviparously throughout the
spring and summer but, after a sexual phase, lay
eggs (the overwintering stage) prior to the onset
of winter. Parthenogenetic reproduction in
which only female offspring are produced is
termed thelytokous parthenogenesis (thelytoky)
- as found in aphids, certain Diptera and a
few Coleoptera; parthenogenetic reproduc-
tion in which unfertilized eggs give rise only to
males is termed arrhenotokous parthenogenesis
(arrhenotoky) - as found in some scale insects,
whiteflies and various Hymenoptera; partheno-
genetic reproduction in which both sexes arise
from unfertilized eggs is termed amphitokous
parthenogenesis (amphitoky) - as found in a
few Thysanoptera. Thysanoptera also exhibit
arrhenotoky and thelytoky. In a few insects, no-
tably certain gall midges, the adult and pupal
stage is omitted from the life-cycle and larvae
give rise parthenogenetically to further larvae;
this process is termed larval paedogenesis. Pupal
paedogenesis occurs where (again, as in certain
gall midges) embryos arise within a 'pupa', which
differs from a 'normal' pupa and is termed a
hemi-pupa (see p. 173).
In the less advanced (hemimetabolous) in-
sects, development through the pre-adult (nym-
phal) stages to adulthood is gradual and usually
involves only partial or incomplete metamorpho-
sis, without a pupal stage (Fig. 7). In the more
advanced (holometabolous) insects, metamor-
Fig. 7 Life-cycle of a hemimetabolous insect, based on a psyllid - family Psyllidae (not to scale).
