Do wasps have glands? If so, then what are wasp glands? Are they similar to human glands? What is their function? By the way, what are human glands? Just being curious, I have made a quick search about glands on the internet. I really wanted to know what people, in general, might think of glands or what the most popular medical sites tell us about glands. Here are a few definitions:

Glands are important organs, you have a variety of them all over your body, and though many of them are small, each produces something important. (source: Kidshealth.org)

A gland is an organ in an animal’s body that synthesizes a substance such as hormones or breast milk for release into the bloodstream (endocrine gland) or into cavities inside the body or its outer surface (exocrine gland). (source: Wikipedia)

… a cell, group of cells, or organ of endothelial origin that selectively removes materials from the blood, concentrates or alters them, and secretes them for further use in the body or for elimination from the body … (source: Merriam-Webster)

I think it is clear that glands are parts of living organisms that secrete/extract something unrelated to the ordinary metabolic need of the gland cells themselves. Glandular products are intended to alter the life or function of a different body part of the same organism or sometimes a different organism.

Traditionally glands are classified into two categories: endocrine and exocrine. Endocrine glands often have a nervous system origin and excrete into the body fluid, whereas exocrine glands are usually epithelial in origin and empty onto the body surface, lumens or cavities. The function of the gland extract is diverse in mammals. Just think of endocrine glands, such as the thyroid or the pituitary gland, and exocrine glands, like the salivary or sweat glands. These structures not only are located in different body parts and develop differently but also produce different compounds with different effects.

What about wasps and other hymenopterans? Like all insects wasps also have glands. Moreover, their glands can also be classified following the same categorization: their endocrine glands are located often in the nervous system and produce hormones that alter their development and circadian rhythm, while exocrine glands are located in the integument (i.e., their “skin”).

The subject of this blog series is: insect glands of the integument. Skin of mammals is densely packed, with four different types of exocrine glands. They are located almost everywhere on the skin and have well known functions and structures:

1. Sebaceous glands produce the waxy sebum that maintains the integrity of the skin barrier and hydrates the outer layer. The sebum contains antioxidants and antimicrobial lipids thereby protecting the body from infections.
2. There are two types of sweat glands:
   1. One covers the body and produces a clear, odorless substance consisting water and table salt (NaCl).
   2. The second type is located around the armpits and perianal area, and they produce a cloudy, odorous, pheromonal excretion that is important in chemical communication (or at least was important before we started to mask them).
3. Mammals also have a unique gland that might have evolved from sweat glands: the mammary glands.

Similarly to the human skin, the integument of Hymenoptera is also rich in glands. For instance, glands are seemingly everywhere in ants (Billen and Delsinne 2013; Billen et al. 2013a). However, there are three facts that make the Hymenoptera exocrine gland system different from the mammalian one:

1. The number of different gland types is much higher in Hymenoptera than in mammals (>75  just in ants; Billen 2009). In fact, ants and other social Hymenoptera are often referred to as “gland factories” (Billen 1998).
2. Most hymenopteran glands have unknown or questionable functions.
3. We think that most of the exocrine glands are yet unknown.

Despite the high number of exocrine glands (EGs) in ants, quite a few have been described in the last year (Billen et al. 2013a–c, Billen and Delsinne 2013). We don’t have an exact number of EGs described from non-social hymenopterans (sawflies and non-aculeate apocritans), but based on our rather rough literature screening ant EGs alone (not counting glands of bees and wasps) easily outnumber those of other Hymenoptera (less than 20; see smart box below).

Box 1. Everything that is known about exocrine glands (n=17) in non-social Hymenoptera. Most of the non-social Hymenoptera endocrine literature deals with two classes of glands: (1) Female accessory glands (Dufour’s gland and venom gland): Cooper 1953 (venom gland Orussus sayii, Orussidae), D’Rozario 1942 (venom gland of various Hymenoptera), James 1926 (venom gland, Dufour’s gland Harmolita sp., Chalcidoidea), Hannah 1934 (venom gland Euchalcidia caryobori, Chalcidoidea), Howard and Baker 2003 (venom gland Bethyilidae, Pteromalidae),  Pampel 1914 (venom and Dufour’s gland Braconidae), Bender 1943 (venom gland Habrobracon juglandis, Braconidae), Quicke et al. 1992, 1997 (venom gland Braconidae), Robertson 1968 (venom gland Perga affinis, Sirex noctilio, Rhyssa persuasoria, Megarhyssa nortoni), Vardal 2006 (venom gland Cynipoidea), Dufour 1841 (venom gland Tenthredo succincta), Bordas 1895 (venom gland Emphytus tibialis and E. cinctus). (2) male accessory glands: Huang et al. 2007 (Cotesia vestalis), Tait 1962 (Perga affinis), Schulmeister 2003 (sawflies), Popovici and Johnson (Platygastridae), D’Rozario (various Hymenoptera). A small number of papers have been published on the male antennal gland of the so called sex segment: Bin et al. 1999 (Braconidae), Bin and Vinson 1986 (Trissolcus, Platygastridae), Romani et al. 2008 (Diapriidae), Guerrieri et al. 2001 (Encyrtidae), Isidoro and Bin 1995 (Amitus, Platygatridae), Isidoro et al. 1999 (Cynipoidea). And an even smaller number of publications treat other glands in the hymenopteran body: abdominal tergal, intertergal, sternal and intersternal glands from Braconidae and sawflies (Quicke 1990, Buckingham and Sharkey 1988) and Chalcidoidea (Lasalle and Polaszek 2000). We have found only one paper on head glands reported from Bethylidae (mandibular gland Gómez et al. 2005).

Exocrine glands play crucial roles in almost all aspects of the social lifestyle (Billen et al. 2013a), but could it really be true that social Hymenoptera have more EGs than non-social Hymenoptera? Looking at the modest number of papers published on EGs of non-social hymenopterans it is possible that this big difference is due to the lack of efforts targeting exocrine glands in the latter group?

Although descriptive taxonomists of non-social taxa often suspect that a gland is located beneath a more densely sculptured patch or a small depression of the cuticle (see Mikó et al. 2010 Figs 5–8), they usually stop at this level and state: “further histological examination is needed”. And really, making the next step, i.e., converting our specimen into a series of sections, is the line passed only very rarely by descriptive taxonomists. We somehow just don’t want to leave our stereomicroscope and go through the painfully difficult fixation, embedding, and sectioning procedure (Is it really so difficult, though? See Sal’s great blog post about this procedure) and especially to reconstruct our specimen which is now spread across 200-300 microscope slides.

Without question, special fixation and transmission electron microscopy is required for the accurate characterization of a gland. The simple confirmation of the exocrine gland’s presence, however, can be useful (e.g., Billen and Delsinne 2013). Non-social Hymenoptera systematics is filled with long lasting questions that could be easily answered by the confirmation of certain glands. For example, what is the function of the Waterston’s organ of Ceraphronoidea? Is an exocrine gland associated with the eucoiline mesoscutellar pit? Do male Nasonia vitripennis specimens actually have the mandibular glands that were predicted by behavioral ecology studies? Is the presence of a metapleural gland unique for ants? With the advent of confocal laser scanning microscopy (CLSM) and other high resolution imaging techniques there is no more need to use histological sectioning for proving the presence of an exocrine gland. With CLSM and some razor blades we can perform the dissections necessary to clarify the presence of a gland more quickly than histology.

We have accidentally discovered quite a few exocrine glands during our anatomical studies on non-social Hymenoptera in the last five years of the Hymenoptera Anatomy Ontology project. These observations give us the impression that the exocrine gland system of Hymenoptera as a whole is understudied. Over the next few weeks we will present an image and a short description of several newly discovered glands.