Type 2 Exocrine Glands in Insects
Above: the area around a large trichoid touch and taste sensory hair or large trichoid sensilla (T) as found
on the antennae of the rove beetles
Aleochara bilineata. Around the base of the socket is a circle of type 1
glands (G!) as described previously (click here) and a single pore opening that belongs to a type 2 gland
(G2). The type 1 glands with their multi-pore clusters also occur elsewhere on the antenna and head (a
couple can be seen at the top of the picture). The type 2 glands on the antenna, however, only occur
beneath the large trichoid hairs, and always singly and beneath the hair (proximal) in the position shown.
This photograph, taken with a scanning electron microscope shows a junction between two antennal
segments. A ring of small trichoid sensilla, of which one is visible here (P) act as proprioceptors.
Proprioceptors sense the position and movement of body parts and these hairs detect bending of the
antennae at the joint between these two segments. Also visible are blunt basiconic pegs which act as
olfactory sensors (O). These pegs are scattered over the surface of the antenna flagellomeres, but are
concentrated in a double ring at the top of each segment at the joint with the next segment further\along, as
shown here. Small trichoid hairs scattered over the surface of the antenna act as touch or tactile sensors
(Ta).

The diagram below shows part of a section through one of the flagellum segments of the antenna.
Above: the antenna is enclosed in a touch cuticle, which forms a hollow cylinder and is shown shaded in grey
to the left of the diagram (the outer most section of the cuticle is visibly layered and called the exocuticle,
whilst the innermost is homogenous and is called the endocuticle). A coeloconic olfactory sensor (C),
consisting of several basiconic pegs in a pit can be seen. Immediately above this are two dendrites running
into two single olfactory basiconic pegs. The diagram emphasises the various glands present. The type 1
unicellular glands have already been described (
click here) and lead to the multi-pore clusters seen in the
topmost photo (G1). The antenna consists of a layer of cells beneath the cuticle (called the
hypodermis (H),
although associated cells may form additional layers). Many of the hypodermal cells are modified to allow the
passage of sensory dendrites through them and some are modified to form type 1 gland cells (G1). Inside the
antenna is a space filled with blood or haemolymph, and in this space sits a large type 2 gland, shown on the
right. This gland and its duct are made up of several cells. The secretory cell (the central gland cell, CGC)
contains a large storage
reservoir (R) into which it releases the chemicals to be secreted. Its dark cytoplasm
contains a dense network of membranous tubes, called endoplasmic reticulum (ER), which help synthesise
and transport the secretions across the cell, and mitochondria (M) are abundant (supplying energy, so this
cell has high energy demands). Densely-staining vesicles (V) probably represent droplets of material to be
secreted and a canal lined with fingerlike projections called microvilli (MV) encloses a cuticular duct (D) which
passes through a second cell (the peripheral gland cell (PGC) or ductule cell) and then through the
hypodermis to reach the outside world via the single pores shown in the scanning electron micrograph above.
This duct conveys the secretions to the outside via this pore. A mass of such gland tissue sends out ducts to
each of the single (G2) pores. This arrangement whereby a reservoir-containing gland cell, a ductule cell and
a hypodermal cell are connected in series to form a gland is common in insect exocrine glands. The reservoir
cells appear to contain several reservoirs and to produce several ducts, though the whole gland contains 6-18
of these cells in each antennal segment. Single cuticular pores also occur elsewhere on the body and they
equate to similar glands underneath the cuticle (whereas the type 1 glands occur on the head and its
appendages only).

The sections below show this central mass of reservoir (R) containing gland cells inside the haemolymph (H)
filled central cavity of the antenna. Click the images to enlarge.
Above (figure 3): left (fig. 3c) - reservoir-containing cells form a H-shaped gland in the haemocoel (blood-filled
cavity) of the antennal segment. Each cell contains more than one cuticular duct (C) (or else one very
convoluted duct). Note the nuclei of the cells (N) which contains the genetic information (DNA). The gland is
well supplied by trachea vessels (T) which carry air and hence oxygen to the cells and carbon dioxide waste
away. The nearby pair of antennal nerves (AN). The scale bar in both figures represents 2 micrometres (2
thousandths of a millimetre). A branch of the antennal nerve (indicated by the asterisk) has become enclosed
by gland tissue and some of these nerve fibres may well innervate the gland which maybe under nervous
control. (In contrast, the type 1 unicellular glands have no visible nervous innervation). At least one of the
cells has fingerlike microvilli (arrow) bordering the haemolymph (H) filled cavity. Microvilli increase the surface
area of a cell and are often sites of export or import across the cell, so it may be that this cell is importing raw
materials (nutrients) from the blood into the gland. All the signs are that these glands are metabolically very
active - they are busy making lots of materials to secrete and this requires lots of energy and nutrients.
Whatever they secrete, it is clearly very important to the insect! Let's look at the structure in more detail:
Under