Minilecture:

Integument and Glands

Presented by D. Merritt

Class II gland cells are derived from epidermal cells but don’t have a duct and don’t contact the cuticle. A common type of Class II gland cell are the oenocytes (pronounced ee-no-sites). In flies, they are clustered in the abdomen, just beneath the cuticle and its associated epidermal cells. They release cuticular hydrocarbons which somehow make their way through the cuticle to be released into the waxy layer. It is believed that the secretion binds to lipophorins that carry it through the hemolymph to be released through the cuticle elsewhere.
In Drosophila, the oenocytes distribution is detectable in strains that express GFP in the cells (right).

J.C.Billener,et al. Nature 461, 987-991 (2009)

Class III gland cells are the secretory cell of a more complex unit called a class III dermal gland. They are diverse in the function of secretions and their locations on the body, but are always associated with cuticle, hence their name. Dermal glands are multicellular, at least during their development. Sometimes there is only one very large secretory cell present but this is probably due to death (apoptosis) of other cells after their developmental job is done. At least one cell secretes a duct that is lined with a cuticle-like substance.
An end apparatus lies in the cavity (reservoir) of the secretory cell. Its role may be to filter or sort the secretions before they enter the duct. The end apparatus tends to fluoresce blue under UV light, suggesting it might be composed of a cutcle-like substance related to resilin.
The secretory cell usually has many microvillae where the secretion is released. In the image at right fine ducts can be seen running from the gland lumen to the reservoirs of each gland unit.

Many class III dermal glands surround the lumen of the colleterial gland of a female sawfly. Image: David Merritt

Dermal glands & the female reproductive system

Review the structure of the female reproductive system of a generalised insect. The female reproductive tract is ectodermal in origin and cuticle-lined from the external opening to the junction of the ovarian ducts.
The cuticle lining of the spermatheca is often sclerotised, perhaps as a way of restricting UV-based damage of the stored sperm. Posterior to the spermathecal duct there is often a pair of female accessory glands. Snodgrass called them colleterial glands because their common function is to secrete a glue that binds eggs together or to the substrate.
The spermathecae have associated dermal glands, either surrounding the capsule itself and releasing directly into the spermatheca, or taking the form of a separate tubule associated closely with the spermatheca.
The female accessory glands usually have many dermal glands producing a secretion and storing it in a reservoir. In just a few cases, the female makes a silk with this material.
Interestingly, in bees the female workers have reduced ovaries because ovarian development is inhibited by the presence of the queen. They retain a well-developed accessory gland and it has become modified to produce venom. So the toxic venom of wasps bees and ants is in fact a modification of the secretion that in most insects is used in association with reproduction.

TOPIC REVIEW

Do you know…?

• the terms for the different layers of cuticle
• how to draw a diagram of the integument, naming the main types of cuticle
• the effect of protein additives on cuticle mechanical features
• the main classes of gland cells and the mechanism for releasing secretion to the outside