LIBRARY

THE ANATOMY OF INSECTS 39
Organs of sound and hearing. - It is generally supposed that insects capable of
producing sound are also able to hear. Organs of sound have previously been
discussed in the chapter on external anatomy and consist of many kinds of
stridulating organs, flight mechanisms, tympanums or drums, and other devices.
The chordotonal organs are variable complex structures which are sound
perceptors or tension receptors and according to Snodgrass (1935) are cellular
elements with or without scolops or sense rods which constitute elongated structures
both of whose ends are attached to the cuticle. johnston's organ, first observed
by Christopher Johnston in 1855, is a specialized cup-like chordotonal
organ occupying much of the second antennal segment in DIPTERA, being
especially well developed in hairy antennre of male mosquitoes and midges
and, to a lesser degree, of bluebottle and greenbottle flies; in LEPIDOPTERA
in satyrid butterflies, and in HYMENOPTERA in paper wasps (Imms). The
tympanal auditory organs on each side of abdominal segment I of grasshoppers,
and on the fore tibire of crickets and katydids, are highly specialized structures
known as the "ears" of these insects.
GLANDULAR SYSTEM
The many variable and important glandular organs of insects are not closely
enough coordinated to constitute a "system" in the real sense of the word, so
this term is used in a very general sense to denote a grouping of miscellaneous
glandular organs of similar structures and functions.
Salivary glands - a variable number located in the head and thoracic regions
which open by means of one to four or more pairs of ducts at the base of the
hypopharynx of adult and larval forms.
The frontal gland of the nasute termite exudes a sticky fluid used as a defensive
and offensive weapon against enemies.
Molting glands - numerous minute unicellular bodies, secreting fluids necessary
for the shedding of the skin during the process of molting in insects exposed
to air, light, and heat.
Silk glands - often a pair of very large tubes, occupying much of the body
cavities of lepidopterous larvre. The silk is drawn through a single spinneret on
the labium. The glands of Filippi are associated with the silk prlfss at the posterior
region of the mouth. In embiids, silk glands and a series of spinnerets occur
in the enlarged first tarsal segment. In certain curculionid and neuropteroid
larvre silk is produced by the Malpighian tubules, obtained from the anus by
means of the mouth parts, and used to construct lace-like cocoons or for other
purposes. The glands of Batelli are hypodermal glands opening into the rectum
and secreting a fluid which is forced from the anus along with air bubbles to
form the froth or spittle of spittle bugs.
Setal glands are very numerous in the larVa! and adults of many insects.
They are small glands with outlets through lumina or tubes of set:e or "tenant
hairs." The venomous stinging and poisonous hairs of certain "woolly" caterpillars
are of this type. Tenant hairs that secrete sticky fluids also occur on the