The Insect Head The insect's head is sometimes referred to as the head-capsule, and is the insect's feeding and sensory centre. It supports the eyes, antennae and jaws of the insect. The upper-mid portion of an insects face is called the 'frons' below this is the 'clypeus' and below this the 'labrum' to either side of which may be seen the edges of the 'mandibles' in some insects various aspects of the 'maxilliary' palps may extend beyond and or below these even when viewed from front on.
The 'frons' = that area of the face below the top two 'ocelli' and above the 'frontoclypeal sulcus' (if and when this is visible) and in between the two 'frontogenal sulci', it supports the 'pharyngeal dilator' muscles and in immature forms it bears the lower two arms of the ecdysial cleavage lines.
The 'clypeus' = that area of the face immediately below the frons (with which it may be fused in the absence of the frontoclypeal sulcus) and the frontoclypeal sulcus. It supports the 'cibarial dilator' muscles and may be divided horizontally into a 'post.' and 'anteclypeus'.
The 'labrum' = is equivalent to the insect's upper lip and is generally moveable, it articulates with the clypeus by means of the 'clypeolabral suture'.
The rest of the front of the head: that bit which is above the frons is known as the 'vertex'; the sides of the head are known as the 'gena'
The Eyes Though some species of insects have been shown to be able respond to light stimulus through their cuticle, most light sensitivity occurs through one or more eyes. Insects possess two different sorts of eyes, the usually large and obviously visible compound eyes, and two varieties of ocelli or simple eyes.
Compound Eyes Compound eyes are so named because the cornea is composed of a number of individual facets or lenses (called ommatidia), rather than a single lens as in ocelli (or our own eyes). The number of separate visual elements or ommatidia varies greatly between species as well as between the larger taxa, so that while worker ants of different species may have between 1 (Ponera punctatissima) and 600 ommatidia per single eye, adult male Odonata may have more than 28,000 per single eye. This creates a considerable difference in the presentation of light stimulus to the insect brain, however the ability of insects to navigate the world by means of visual stimuli suggest that they have overcome the problems inherent in this multi-faceted perception.
Much like our eyes, the eyes of insects, can be divided into four basic parts: the supportative material that keeps all the parts together; a light gathering part (the lens and the auxilary lens called a 'crystalline cone'); a light receptor that converts the recieved light into electrical energy; and the nerves that carry the electrical impulses to the brain for analysis. In the compound eyes of insects these parts are repeated numerous times side by side in a space saving hexagonal pattern.
The lens is formed by a transparent and colourless cuticle and it is usually biconvex. Beneath this is the crystalline cone (which is comprised of four cells called 'Semper cells' after the man who first described them). Normally this functions as a secondary lens. The receptive parts of an insect's eye are the 'retinula cells'. Each ommatidium normally has eight retinula cells arranged to leave a central core space in the centre of the ommatidium, into which each retinula cell projects a series of microvilli (like very small fingers). These microvilli are the actual light detecting part of the cells and are collectively referred to as the rhabdomere (think cornea). The eight (or occasionally 7 or 9) rhabdomeres (sets of microvilli) form a rhabdom.
The corneal lens is supported by 'primary pigment cells' and the retinula cells and associated rhabdoms are supported by 'secondary pigment cells'. The retinula cells are connected to axons at the base of the eye, it is these which carry the information collected by the lenses and converted into electrical impulses by the rhabdom to the brain, thus allowing the insect to see.
Ocelli are present in most insects to some degree, though as with all aspects of insect anatomy there is a great deal of variety in form and even in relative function. Generally they consist of five separate parts the 'cornea', the 'corneagen layer', the 'retina', the 'pigment cells', and the 'central nervous connections'.
1)The CornealLens this is a thickened area of generally transparent cuticle to the outside of the ocellus which serves as a lens.
2)The Corneagen Layer this is a single layer of specialised transparent and colourless epidermal cells which secrete the cornea.
3)The Retina this is a group of primary sensory cells which convert light into an electrical stimulus and transfer it to the; the cells are called 'retinula' cells and they are arranged in circular groups with each member of the group contributing to its portion rhabdomere to the group rhabdom. The rhabdom is the light sensitive pigment, or the part of the ocellus that converts the light into an electrical stimulus.
4)The Axon, which is the nerve link to the 'protocerebrum' and hence to the 'Corpora pedunculata' (the brain) which in turn allows the insect to use the information the ocellus produces.
5)The Pigment Cells this is a group of highly pigmented (coloured) cells variably distributed around the ocellus whose main roll would appear to be the exclusion of light from parts of the ocellus other than the camera. The function of the corneal lens is obscure, although it does project an image into the ocellus this image forms below the level of the light-sensitive cells, or rhabdom. Therefore the ocellus can generate no image information, however it is very sensitive to low levels of light and to changes in light intensity and scientists believe that the ocelli are useful in allowing the insect to detect the horizon, to respond quickly to changes in light intensity.
Two different forms of ocelli have been described for insects, Dorsal ocelli and Lateral ocelli.
Dorsal ocelli occur mostly in adult insects and are situated on the front of the insects face in the area of the 'frons' and or the 'epicranium', lateral ocelli generally occur on the sides of the insect head and are the form of eye most common in larval forms; there are a number of concrete differences between the two forms which can be be found explained in any competent entomological text book such as Imm's 1984.
The second (middle) tagma of an insect's body is called the thorax. This region is almost exclusively adapted for locomotion -- it contains three pairs of walking legs and, in many adult insects, one or two pairs of wings.
Structurally, the thorax is composed of three body segments: prothorax, mesothorax, and metathorax. These segments are joined together rigidly to form a "box" that houses the musculature for the legs and wings. Each segment has a dorsal sclerite, the notum (pronotum, mesonotum, and metanotum) which may be further subdivided into an anterior scutumand a posterior scutellum. The ventral sclerite of each segment is the sternum (prosternum, mesosternum, and metasternum).The side of each segment is called the pleuron -- it is usually divided by a pleural suture into at least two sclerites:; an anterior episternum and a posterior epimeron.
The pleural suture marks the location of an internal ridge of exoskeleton (an apodeme) that strengthens the sides of the thorax. Ventrally, this apodeme forms a point of articulation with the basal leg segment (the coxa). In thoracic segments that bear wings, the pleural apodeme runs dorsally into the pleural wing process, a finger-like sclerite that serves as a pivot or fulcrum for the base of the wing.
A special "strut" of exoskeleton reinforces the ventral corners of each thoracic segment and provides a rigid site for attachment of leg muscles and ventral longitudinal muscles.This structure, called the furca, forms during development when a pair of sternal apophyses fuse internally with the ridge (apodeme) from each pleural suture. The points of invagination are often visible externally as furcal pits located near the midline of the sternum (and often joined by a sternacostal suture). This internal "brace" mechanism is similar in structure to the tentorium which serves a related function inside the head capsule.