The Insect Head
Entomologists’ believe the insect head consists of six segments, segments that have evolved from an ancestral precursor to form the features we see today. The head consists of the head capsule, the mouthparts, two compound eyes and two antenna (Fig.1). The head is connected to the thorax by the cervical membrane (neck) which is flexible and allows head movement.
The Capsule – is divided into several regions as defined by sutures, sulci and sclerites; these regions vary in location and visibility in different insects. The apex of the head capsule is the vertex – in many insects the ocelli will be in this area. The region behind (or posterior) the vertex is the occiput, defined by the occipital suture and the postoccipital suture. The postoccipital suture surrounds the foramen magnum (sometimes called the occipital foramen), the posterior opening of the head capsule (fig. 3). This opening is covered with the flexible cervix – the insect neck.
The front (anterior) of the head capsule has a suture line between the compound eyes, below the median ocellus, called the coronal suture. Meeting the coronal suture to form an inverted Y are the frontal sutures, which frame the top of the frons. Together, the coronal and frontal sutures form the epicranial suture, which can also indicate ‘lines of weakness’ – lines where the integument will split to allow for molting.
On either side of the frons and below the compound eyes are the gena – the insect equivalent of our cheeks. The lines below the gena are the subgenal sutures. Below the frons is a sclerite called the clypeus, and below the clypeus lies another sclerite, the labrum – the “upper lip”.
The Mouthparts (See Dr John Meyer’s Mouthparts Tutorial) – Insects differ from vertebrates in that the mouthparts are external to the oral cavity (the ‘Ectognatha’). Orthopterans are hypognathous (“under jaw”), that is, the mouthparts are ventrad of a vertically oriented head. The position of the jaw in insects is either hypognathous, prognathus (“front jaw”), such as in beetles or opisthognathous (“behind jaw”), such as in most Hemiptera.
The clypeus and the labrum are connected with a flexible membrane which allows for movement. Below these sclerites are the mandibles – paired unsegmented and heavily sclerotized jaws that have both an incisor and a molar region for cutting and grinding plant matter. Each mandible is hinged at two condyles on the head capsule. There are two points of muscle attachment: an anterior secondary articulation and a posterior articulation at the postgena. The muscles are attached to the top of the head (see inside the occipital foramen, fig. 3 below right), the muscle which opens the mandible is called the abductor, and the stronger muscle that closes the mandible is called the adductor.
Below the mandibles are a pair of mobile maxillae. The first basal segment of the maxillae is the cardo, which connects to the stipes, which in turn has paired lobes, the mesal sclerotized and toothed lacinea and a palp-like galea, both of which help manipulate and break-up food particles. Also attached to the stipes is lateral sclerite called the palpifer, the basal segment to a further 5-segmented maxillary palp.
The labium lies medial of the maxillae, ventral of the mandibles. At the base of the labium is the hypopharynx, a tongue-like lobe that lies in the preoral cavity. Anterior to the hypopharynx is the basal submentum, the palp-bearing prementum and the distal ligula with two pairs of apical lobes – the medial glossa and the lateral paraglossa.
The conehead is representative of mandibulate insects, which includes major orders such as the Orthoptera (grasshoppers, crickets, katydids etc), Blattodea (cockroaches), Mantodea (mantids), Coleoptera (beetles), Hymenoptera (bees, ants, wasps, sawflies), Isoptera (termites, Odonata (dragonflies, damselflies), Neuroptera (lacewings, antlions) and the larva of Lepidoptera (moths, butterflies). They all have mandibles capable of lateral movement and are used for biting and chewing.
The haustellate insects have modified mouthparts which suck in foods by piercing, siphoning or sponging. Piercing mouthparts have stylets, modifications of the mandibles and/or the maxillae that are capable of piercing or abrading animal or plant tissue. Once the tissue is penetrated, fluid can be imbibed by active sucking and/or by taking advantage of hydrostatic pressure in plant phloem. The piercing and sucking method of feeding is used by mosquitoes (Diptera), fleas (Siphonoptera) and the true bugs (Hemiptera)
Lepidoptera feed by siphoning with a proboscis which is an adaptation of galeae of the maxillae, which form together to form a food canal. The proboscis can be coiled when the insect is at rest. The other mouthparts are vestigial or entirely missing.
Many flies without stylets feed by sponging or lapping, which is done with a modified labellum. The labellar lobes are pressed down on the fluid surface from which the fly will feed, and channels (pseudotrachae) in the lobe direct food to the food canal. Some flies will make use of prestomal teeth in the labellum to rasp food and they can also eject saliva to help liquefy food particles. Fluids are drawn up by the cibarial and pharyngeal pump to the esophagus.
Bees have both mandibles and sucking mouthparts. The mandibles are used for pollen and wax manipulation, while the glossae of the labium are elongated and fused to form a ventrally open canal. This is surrounded by the galeae and labial palps to complete the food canal.
Differences in the mouthparts are often critical in identifying certain insects, and these differences will be indicated in the sections on insect diversity.
Comments, critiques and corrections are encouraged.
Gordh G. and D.H. Headrick. A Dictionary of Entomology. Cabi 2001.
Romoser, William S. The Science of Entomology, pp. 26-49. Collier-MacMillan 1973.
Wallace, Robert L. et al. Beck and Braithwaite’s Invertebrate Zoology, 4th Ed., pp. 248-250. MacMillan 1989.
Resh, Vincent H. and R. T. Cardé, Eds. Encyclopedia of Insects, pp. 15-19, 750-755. Elsevier 2003.
All photographs © Adrian Thysse 2011