Insect head:- it’s origin, structure, function and modification in different order of insect
- 1. Insect head:- it’s origin, structure, function and modification in different order of insect . . .
- 2. The insect head is a hard, highly sclerotized compact capsule bearing a group of feeding appendages around the mouth, sensory organs and internally a major centre of neuroendocrine coordination
- 3. Head Segmentation • According to phylogenetic considerations, the insect head is in fact a product of evolution and can be termed as Syncephalon. • It consists of two parts. An anterior procephalon and a posterior gnathocephalon. PROCEPHALON • The procephalon represents preoral region of the head and bears two paired structures, the eyes and antennae. • The procephalon is thought to be derived from the ancestral head, the archencephalon or blastocephalon, after an incorporation of at least one posterior trunk segment of the ancestor. • The brain is considered as a fused ganglion of the procephalon.
- 4. GNATHOCEPHALON • The gnathocephalon represents postoral region of the head. • It bears three paired structures; the mandible, first maxillae and second maxillae or labium. It is mostly developed in order to equip the head with feeding appendages. • Embryological and morphological studies suggest that the gnathocephalon is composed of three segments, viz., the mandibular, maxillary and labial segments. • In an adult insect, these segments are evident from their paired appendages. • The suboesophageal ganglion is undoubtedly, a composite ganglion of the gnathocephalon. • Based on the embryological studies, the segments of insect head are six such as: 1. Pre antennal segment 2. Antennal segment 3. Intercalary segment 4. Mandibular segment 5. Maxillary segment 6. Labial segment
- 6. 1. Pre-antennary segment: It is recognized by its neuromere that forms procerebrum of the brain and embryonic appendages. 2. Antennary segment: It is indicated by its neuromere that forms deuterocerebrum of the brain and its appendages are antennae. 3. Intercalary segment: Intercalary segment is with feeding appendages which are homologous with antennae of crustacean and its neuromere, tritocerebrum. Traces of this segment are found in most embryos. 4. Mandibular segment: It is indicated by its neuromere, the mandibular ganglion and its appendages the first maxillae. 5. Maxillary segment: It is recognized by its neuromere, the maxillary ganglion and its appendages the first maxillae. 6. Labial segment: Labial segment in with its appendages the labium and neuromere, labial ganglion. The neuromeres of the last three segments fuse during development to form the sub-oesophagial ganglion.
- 7. HEAD SKELETON • The head of the insect loses its primary segmentation during the post-embryonic development. • Due to enormous sclerotization, the head becomes a hard skeletal capsule. • It bears externally a set of sensory and gnathal appendages. • The sense organs include a pair of antennae, a pair of compound eyes and two or three ocelli. • The gnathal appendages comprise a pair of mandibles, a pair of maxillae and a labium. • The gnathal appendages constitute collectively along the labrum and hypopharynx, a well elaborated feeding apparatus. • The position of mouth parts in relation to the body axis differs in different group of insects. The mouth parts may project downward, forward or backward and the head, accordingly, is designated as the following types.
- 8. Hypognathous Head: • The long axis of the head is vertical and mouthparts are ventral i.e., pointing downwards (primitive type). The occipital foramen lies in or near transverse plane. Ex: Grasshoppers, cockroach. Prognathous Head: • The long axis of the head is horizontal, mouthparts are anterior in position occipital foramen is slightly inclined. Ex: Soldiers of termites and larvae of endopterygota
- 9. Opisthognathous Head: • Occurs in Homoptera and Hemiptera where the head is deflexed backwards so that mouthparts / proboscis slopes backwards between the front legs. HEAD SUTURES The head capsule of the matured insects is differentiated into several regions by the sutures. The sutures are nothing but the secondarily developed fine groves. They are inflections or merely external impressions of the internal stiff cuticular ridges which provide mechanical support to the cranial wall. These sutures are the product of sclerotization and except the post-occipital suture, others have no metameric segmentation.
- 10. The Clypeo-labral suture: This suture represents the lower margin of the clypeus. The labrum hanging down from the suture. It bears a prominent ridge internally. The ridge possesses an apodeme at the centre. The apodeme provides an insertion to the anterior retractor muscles of the labrum. The Clypeo-frontal or Epistomal suture: It occurs between the clypeus and the frons. It bears laterally the anterior tentorial pits. It internally bears a strong inflection. Those inflection strengthens the lower part of the head against the stretching of the mandibles.
- 11. The Epircanial suture: It is a composite suture. It appears in the form of an inverted-Y above the facial region in most of the insects. The stem is known as the coronal and lateral arms, the frontal sutures. The epicranial suture is partially or even completely reduced in some insects.It is totally lacking from the head of apterygotes. Snodgrass (1935) and Du porte (1957) stated that the epicranial suture represents merely the line of weakness and can be said in true sense as the ecdysial suture.It does not bear the skeletal ridge on the inner surface and therefore does not provide either a site for muscle attachment or mechanical support to the head capsule.
- 12. The Occipetal suture: It is horse-shoe or U-shaped suture, and well evident on the posterior part of the head of the orthopteroid insects.It starts from posterior articulations of the mandibles of either side.Both the arms after running a long distance and join one another above the occipital foramen.It seperates the genal area from the post-genal area of the lateral region of the head.It also differentiates the vertex from the occiput in posterior region of the head. It forms an internal ridge strengthening the posterior epicranial wall.
- 13. The post-occiput suture: It embraces the occipital foramen magnum from the dorsal and lateral sides. Each end of the suture merges in the hypostomal suture in ventral region.This site is well defined due to the presence of the posterior tentorial pits.It possesses internally a strong epidermal ridge, the post-occipetal ridge. It provides a site for the attachment of prothoracic muscles which move a pair of ventro-lateral condyles. The neck membrane is firmly attached to this ridge.The condyles articulate with the neck sclerites. It is the only suture having the metameric significance. It alone marks the fusion of the maxillary and labial segments of the head.
- 14. The Frontogenal or subocular suture: It develops on each lateral region of the head. It starts from the junction of the clypeofrontal and the subgenal sutures.It terminates in the close vicinity of the lower margin of each compound eye. It is virtually well evident in the orthopteroid insects. The Subgenal Suture: These sutures occur on the lateral sides of the head.Each suture joins the posterior tentorial pit with the anterior tentorial pit of the same side.Each suture is divisible into two parts. One part of the subgenal suture is called the hypostomal suture. The hypostomal suture runs from the posterior tentorial pit up to the posterior articulation of the mandible.
- 15. The Ocular suture : These sutures are the annular grooves around the compound eyes. They form the skeletal ridges internally. The Antennal Sutures: The marginal depressed ring around the antennal sockets are known as the antennal sutures.Each suture gives internally the submarginal ridge.The submarginal ridge in a large number of insects bears a pivot like process known as the antennifer. The antennifer provides articulation to the antenna.
- 16. HEAD SCLERITES The sclerites represent simply, the inter sutural areas. The head of the generalized insect is typically exhibiting the following sclerites. 1. The labrum 2. The clypeus 3. The frons 4. The epicranium 5. The occiput 6. The post-occiput 7. The Genae and subgenal areas 8. The Ocular sclerites
- 19. Tentorium: It is an endoskeleton structure.It is composed of three pairs of arms, the anterior, posterior and dorsal tentorial arms.The anterior tentorial arms develop from the anterior tentorial pits in the subgenal or epistomal suture.The posterior tentorial arms arise from the posterior tentorial pits in the lower ends of the post-occipital suture.The dorsal tentorial arms take origin in the form of secondary out-growths of the anterior arms.All these tentorial arms unite in the middle forming an expanded central skeletal platform.It may simply constitute a structure resembling the X. The presence of a broad tentorial plate occurs commonly in the pterygote insects and is commonly known as the corporotentorium. The anterior tentorial pits: The anterior tentorial arms of the tentorium arise from some part of subgena or epistomal suture. The posterior tentorial pits: The posterior tentorial arms of the tentorium arise from ventral ends of post occipital suture and lie in the lower extremities of post occipital suture
- 20. Modifications in head capsule: The anterior frontoclypeal and posterior, lateral as well as ventral regions of the head capsule are greatly modified in some insects. 1.Fronto-clypeal Region: The epistomal suture is wanting in cockroaches and some other insects in which the clypeal region can be separated from the frontal region by the position of anterior tentorial pits.Sometimes the frontal sutures are also indistinct.In the Hemiptera, Hymenoptera and Lepidopteran larvae, the frons is greatly suppressed.In these insects, on the basis of position of the dilators of cibarium or that of buccal cavity, the clypeal region can be recognised.Similarly the frontal region can be identified by the position of a median ocellus and origin of labral muscles. 2.Hypostomal bridge: In the lepidopteran larvae, the hypostomal lobes do not unite together while in other insects, they fuse at the middle forming a complete hypostomal bridge in order to close the foramen magnum ventrally
- 21. 3.Gula: It is a ventral sclerotic plate of the head capsule separating the foramen magnum from the base of the labium.It is well developed in the insects having prognathous head in order to place the mouth parts in forward direction.The gula is separated from the laterally expanded postgenae by the gular structures.The gular sutures are in fact modified postoccipital sutures. The gula varies in length and width in different species of Coleopteran and Neuroptera. References: O. W. RICHARDS & R. G. DAVIES IMMS' GENERAL TEXTBOOK OF ENTOMOLOGY Volume 2. The Order of insect: page no 421-1175 R. F. Chapman Insect structure and function R. E. Snodgrass Principles of Insect Morphology TNAU Textbook of Insect Morphology and Systematics prepared Ch.Naga Satyasri : THE INSECT HEAD