and more than 10 subfamilies. It compromises of beetles with specialized body structures for performing a specific function. Members of this suborder are collectively known as adephagan, and include ground beetles (including tiger beetles) and a variety of aquatic beetles, such as predaceous diving beetles and whirligig beetles. Most of the species belong to the family Carabidae (ground beetles). Their body structures are specialized for Defense, reproduction, grooming, predatory habits and sometimes even multipurposed organs.
All families of adephagan have paired pygidial glands located posterodorsally in the abdomen, which are used for secreting chemicals. The glands consist of complex invaginations of the cuticle lined with epidermal cells contiguous with the integument. The glands have no connection with the rectum and open on the eighth abdominal tergum. Secretions pass from the secretory lobes, which are aggregations of secretory cells, through a tube to a reservoir lined with muscles. This reservoir then narrows to a tube leading to an opening valve. The secretory lobes differ structurally from one taxon to another; it may be elongated or oval, branched basally or apically, or unbranched.
Specialized body parts
The secretion is realized in one of these manners:
- Oozing: if the gland is not muscle-lined, the discharge is limited in amount.
- Spraying: if the gland is muscle-lined, which is typically the case of carabids, the substances are ejected more or less forcefully.
- Crepitation is only associated with the Brachininae carabids and several related species. See bombardier beetle for a detailed description of the mechanism. Theyuse such mechanisms as a highly specialized defense.
The secretions differ in the chemical constituents, according to the taxa. Gyrinids, for instance, secrete norsesquiterpenes such as gyrinidal, isogyrinidal, gyrinidione, or gyrinidone. Dytiscids discharge aromatic aldehydes, esters, and acids, especially benzoic acid. Carabids typically produce carboxylic acids, particularly formic acid, methacrylic acid, and tiglic acid, but also aliphatic ketones, saturated esters, phenols, aromatic aldehydes, and quinones. Accessory glands or modified structures are present in some taxa: the Dytiscidae and Hygrobiidae also possess paired prothoracic glands secreting steroids; and the Gyrinidae are unique in the extended shape of the external opening of the pygidial gland.
The function of many compounds remain unknown, yet several hypotheses have been advanced:
- As toxins or deterrent against predators, some compounds indirectly play this role by easing the penetration of the deterrent into the predator's integument.
- Antimicrobial and antifungal agents (especially in Hydradephaga)
- A means to increase wetability of the integument (especially in Hydradephaga)
- Alarm pheromones (especially in Gyrinidae)
- Propellant on water surfaces (especially in Gyrinidae)
- Conditioning plant tissues associated with oviposition