*COMMENT ~ Orders: Character notes. 
 
*CHARACTER NOTES 
#1. ~ (alternatively) is here used to indicate sometimes not
unreasonably included in or reduced to.
#6. Note the essential distinction to be made between parasitic, referring
to the general life-style of a species (q.v.); and the same term when it
relates, as here, to the precise feeding habits of individuals. The different
concepts are often confused in entomological literature. See the character
concerned with the feeding habits of larvae for further comments. \par{}In
practice, the distinction between parasitic and predatory is less clear
than text-book definitions usually suggest. The larvae of many Diptera and
Hymenoptera, in particular, contradict the common assertion that highly
specialised parasitism does not result in the unduly premature death of the
host (with the added, teleological supposition that to do so is inimical to
the long term survival of the parasitic species). In fact, many dipteran and
hymenopteran larvae universally described as parasitic are highly adapted to
keep the host alive only for as long as it is needed as a source of food,
before ultimately killing it. 
#15. Gnathy refers to the inclination of the long axis of the head, and the
position of the mouthparts, relative to the long axis of the body.
\par{}Prognathous: head aligned more or less horizontally with the long axis
of the body, the mouthparts more or less anterior. \par{}Hypognathous: long
axis of the head more or less vertical, the mouthparts more or less ventral
(the common condition). \par{}Opisthognathous: head inclined beyond the
vertical against the body, so that the chin recedes (an uncommon state). 
#17-20. For more details on the interpretations of insect mouthparts, see the
Notes attached to the character concerned with whether the trophi are of
generalized or highly specialized type. \par{}The following terminology
relates the diversity of insect mouthparts to the generalized biting type,
represented on the illustration by those of a beetle (\i{}Dytiscus\i0{},
ventral view of the right side of the head). See also the diagrams accessible
via the Insect morphology toolbar button. \par{}The LABRUM (upper lip) is a
single, sclerotized plate, hinged on the adjoining part (\i{}clypeus\i0{}) of
the rigid head capsule, overlying the opening of the mouth and the bases of
the other mouthparts. Its inner surface has gustatory functions, and in
Hymenoptera it bears a small lobe (the \i{}epipharynx\i0{}), which assumes
impressive proportions as the epipharyngeal stylet in the fleas. \par{}The
two MANDIBLES (jaws, concerned with biting and chewing) lie over the mouth,
and articulate at the sides of the rigid head capsule. They are always
relatively simple and unsegmented, although they are sometimes much modified
in size and form. They may represent the bases of a pair of ancestral limbs.
\par{}The HYPOPHARYNX is an unpaired, medial, tongue-like organ, arising from
the floor of the mouth and projecting from between the oral cavity and the
labium. It is inconspicuous in mouthparts of the generalized type, but is
highly developed in some groups. \par{}The MAXILLAE (lower, accessory jaws)
are relatively complex, with a principal axis comprising an articulating basal
segment (the \i{}cardo\i0{}), and a main axial segment (the \i{}stipes\i0{}),
the latter bearing a segmented (sensory) \i{}maxillary palp\i0{}, an axial
maxillary lobe (the \i{}lacinia\i0{}), and a lateral lobe (the
\i{}galea\i0{}). \par{}The bilaterally symmetrical LABIUM, functioning as the
lower lip, represents a second pair of maxillae that have become fused in the
course of evolution. It exhibits a \i{}mentum\i0{} and a \i{}prementum\i0{}
(representing fused cardo and stipes of the maxillae); and more distally on
each side, a medial \i{}glossa\i0{} (representing the maxillary lacinia), a
\i{}paraglossa\i0{} (outside it, representing the maxillary galea), and a
\i{}labial palp\i0{}. Differing extents of fusion of the components of the
labium are exhibited among the insect Orders, as exemplified in the
accompanying images. 
#21. The generalized biting type is adequately exemplified in the
accompanying illustrations by the mouthparts of \i{}Dytiscus\i0{}, but some
insect Orders exhibit or are characterised by spectacular modifications
associated with different life-styles and feeding habits: modifications
involving fusion of components, reduction or loss of some, and/or major
re-shaping of others. For the present purpose, we are adopting the convenient
and helpful standard practice of categorizing the diversity of insect
mouthparts with reference to the structures identifiable in the generalized
biting type. \par{}Comparative morphological and phylogenetic interpretation
of the enormous range of known insects show that they can all be plausibly
derived from a myriapodan ancestral form exhibiting twenty similar segments,
by invoking co-ordination and/or amalgamation of segments and their paired
appendages to perform related functions. Hence the head reflects adaptations
for sensory perceptions and feeding. It seems to represent three pre-oral
segments (bearing the antennae and the eyes), followed by three fused,
appendage-bearing segments constituting the mouthparts. The latter are flagged
in sequence by their paired appendages; viz., the \i{}mandibles\i0{} (over the
oral cavity, and articulating immediately behind the \i{}labrum\i0{}); the
\i{}maxillae\i0{} (which afford more evidence of phylogenetic derivation from
walking legs); and the \i{}labium\i0{}, apparently representing a second pair
of maxillae that have usually become fused into single structure, but
incomplete fusion attesting to its dual origin can be seen in some Orders.
\par{}The standard interpretations of mouthparts components in the
accompanying illustrations may suggest outrageous phylogenetic theorizing when
presented without the underlying evidence from comparative morphology and
world-wide sampling, but they are in fact well substantiated. Furthermore,
students of the history of science should realize that the principal
descriptive terms, the comparative morphological concepts, and the main
classificatory conclusions derived from contemplating insect mouthparts,
although presented in overtly phylogenetic terms in modern textbooks, were
originally formulated without resort to evolutionary considerations. The
taxonomic practices of late Eighteenth and early Nineteenth Century
entomologists were apparently uninfluenced by the evolutionary insights of
Lamarck, and their classificatory legacy has remained largely impervious to
Darwin's subsequent, more influential exposition. The realization of the truth
of evolution represented a major advance in human thought (giving rise, for
example, to the science of genetics); but it is a popular fallacy that the
concept had a revolutionary effect on taxonomic practice. In fact, it was the
inevitable consequence of contemplating the taxonomic facts set out so
elegantly in the works of early taxonomists, exemplified here by John Curtiss
\i{}British Entomology\i0{}. \par{}The following terminology relates the
diversity of insect mouthparts to the generalized biting type, represented
on the illustration by those of a beetle (\i{}Dytiscus\i0{}, ventral view of
the right side of the head). See also the diagrams accessible via the Insect
morphology toolbar button. \par{}The LABRUM (upper lip) is a single,
sclerotized plate, hinged on the adjoining part (\i{}clypeus\i0{}) of the
rigid head capsule, overlying the opening of the mouth and the bases of the
other mouthparts. Its inner surface has gustatory functions, and in
Hymenoptera it bears a small lobe (the \i{}epipharynx\i0{}), which assumes
impressive proportions as the epipharyngeal stylet in the fleas. \par{}The
two MANDIBLES (jaws, concerned with biting and chewing) lie over the mouth,
and articulate at the sides of the rigid head capsule. They are always
relatively simple and unsegmented, although they are sometimes much modified
in size and form. They may represent the bases of a pair of ancestral limbs.
\par{}The HYPOPHARYNX is an unpaired, medial, tongue-like organ, arising from
the floor of the mouth and projecting from between the oral cavity and the
labium. It is inconspicuous in mouthparts of the generalized type, but is
highly developed in some groups. \par{}The MAXILLAE (lower, accessory jaws)
are relatively complex, with a principal axis comprising an articulating basal
segment (the \i{}cardo\i0{}), and a main axial segment (the \i{}stipes\i0{}),
the latter bearing a segmented (sensory) \i{}maxillary palp \i0{}, an axial
maxillary lobe (the \i{}lacinia\i0{}), and a lateral lobe (the
\i{}galea\i0{}). \par{}The bilaterally symmetrical LABIUM, functioning as the
lower lip, represents a second pair of maxillae that have become fused in the
course of evolution. It exhibits a \i{}mentum\i0{} and a \i{}prementum\i0{}
(representing fused cardo and stipes of the maxillae); and more distally on
each side, a medial \i{}glossa\i0{} (representing the maxillary lacinia), a
\i{}paraglossa\i0{} (outside it, representing the maxillary galea), and a
\i{}labial palp\i0{}. Differing extents of fusion of the components of the
labium are exhibited among the insect Orders, as exemplified in the
accompanying images. 
#27. Specialized inhabitants of dark habitats, with the compound eyes
vestigial or lacking, are recorded from other parts of the world for several
Orders - notably from Hymenoptera (a few ants), Dermaptera, and Hemiptera -
which, in the absence of evidence to the contrary, are tentatively assumed to
universally exhibit them in Britain. 
#40. Corneous dots (nygmata, thyridia): small, thickened, chitinous (usually
semi-transparent, whitish) spots, of almost universal occurrence in
Trichoptera, and said by Riek (1970) to be common also in Neuroptera.
Possibly due to presence of a gland or sensory organ (Imms, 1957). 
#47. Where the number of tarsi is known to differ in fore-, mid- and
hind-legs, the data in this data set are intended to be inclusive. In using
the character for identification, however, while it should suffice to enter a
count for one leg, it would be safer to enter an observed range. 
#50. Cerci: paired appendages on the 11th abdominal segment, which when
present vary from short bristles to elongated, tenuous filaments to large
pincers. 
#57. Note the essential distinction to be made between parasitic, referring
to the general life-style of a species (q.v.); and the same term when it
relates, as here, to the precise feeding habits of individuals. The different
concepts are often confused in entomological literature. \par{}In practice,
the distinction between parasitic and predatory is less clear than
text-book definitions usually suggest. The larvae of many Diptera and
Hymenoptera, in particular, contradict the common assertion that highly
specialised parasitism does not result in the unduly premature death of the
host (with the added, teleological supposition that to do so is inimical to
the long term survival of the parasitic species). In fact, many dipteran and
hymenopteran larvae universally described as parasitic are highly adapted to
keep the host alive only for as long as it is needed as a source of food,
before ultimately killing it. 
#67. Puparium: a persistent, hardened and modified, usually barrel-shaped,
ovoid or globular derivative of the cuticle of the third larval instar,
functioning as a cocoon enclosing a thin-skinned pupa. In a few families (e.g.
\i{}Stratiomyidae\i0{}), the last larval skin persists in unmodified form
(transitional puparia). 
 
