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• Mouthparts 1 • Mouthparts 2 • Thorax and Abdomen 1 • Thorax And • HeadHead •• MouthpartsMouthparts 11 •• MouthpartsMouthparts 22 •• ThoraxThorax andand abdomenabdomen 11 •• ThoraxThorax andand abdomenabdomen 22 •• CockroachCockroach dissectiondissection © 1997 B.K. Mitchell & J.S. Scott, Department of Biological Sciences, University of Alberta The compound eyes are often the most COCKROACH BRAIN - dorsal aspect The inside of the prominent structures on the insect head. Drawing of dissected cockroach head contains more The Insect Head Adult holometabolous insects,as well as head showing brain and related than the brain, of immatures and adults of hemimetabolous Insects are strongly cephalized animals, that is, many of the important nerves. Frontal and hypocerebral course. There are insects have them. Insect compound ganglia are part of the stomatogas- many muscles that functions are moved anteriorly with a high degree of merging or eyes have thousands of more or less tric nervous system, while the cor- operate the various condensing of segments, sensory structures and neural ganglia. This equivalent sensory cartridges called om- pora cardiaca and corpora allata are appendages - the module illustrates the preceding statement. Additional information on matidia. Each ommatidium has a hexago- mouthpart muscles the insect head can be found in the mouthpart module. nal lens (hundreds in focus in this picture) being particularly and six to eight light-sensitive cells. Sin- complex. Six or seven segments are condensed to form the head capsule. This gle homologous sensory cells from nu- strong structure provides protection for the brain, support for eyes, oesophagus frontal ganglion merous adjacent ommatidia respond to This cleared whole ocelli, antennae and mouthparts. The strongest muscles in the head light in their limited field of view and send mount reveals an- serve the mandibles in chewing insects and the sucking pump in the information to the same place in the recurrent nerve other aspect of the piercing-sucking insects. optic lobe of the brain. The image formed head that is often in the insect brain is not as detailed as antennal nerve The hard exoskeleton that is a common feature of arthropods is overlooked - the ours but insects can be exquisitely sensi- optic lobe tentorium (red ar- particularly well illustrated in the head. The lesser appreciated internal tive to movement in the visual field. They rows). cuticular support structures are also well represented by the cross- also can see in colour. corpus cardiacum bracing tentorium. corpus allatum The tentorium is the “internal skeleton” which gives strength to the head capsule and provides a place for muscle attachment. It is formed by tube-shaped inward ex- Supported in part by B.K.Mitchell and J.S.Scott Academic Technologies for Learning © Department of Biological Sciences hypocerebral tensions of the exoskeleton. The mandibles, maxillary and labial palpi are all and Faculty of Science, University of Alberta University of Alberta 4 ganglion 8 visible in this frontal view. 13 Grasshoppers and cockroaches This section through a compound This is a scanning (drawings) are often used to illus- retinular layer eye shows some of the features. electron micro- trate the most basic form of the in- The lens layer is in red, the eye graph of the right sect head. The eye is often promi- lens retinular layer in black and gray side of a bug em- nent and the mouthparts can seem layer and the first of three parts of the bryo. strange to us. The evolutionary optic lobe in blue-gray. flexibility of the four mouthpart seg- Can you find the mental appendages is the theme of The retinular layer contains the following seg- the mouthpart segment of this se- sensory cells (six to eight per om- ments? ries. matidium). The dark layer just un- der the lens layer contains pig- In cockroaches, the head ments that help reduce light scat- • labial Back View is very flexibly attached to tering during the day. the thorax via an exten- • labral sive neck membrane (arrow). The oesophagus, optic • mandibular * heart, tracheal system, lobe and nervous system all •maxillary pass through the foramen at the back of the head The green arrow marks the boundary between the head and thorax. ( asterisk ). The eye is the bulge on the side of the head. Ventral is down and anterior is to the right. This is a dark adapted eye so the pigments are all at one end. This increases 1 light scattering (decreases resolution) but it increases sensitivity. 5 9 Insect heads take many shapes reflecting the characteristic feeding habits of the Jake Remple, a Cana- The sub- species concerned. For example, predaceous insects have forward directed dian entomologist, HEAD OF INSECT oesophagus oesophageal mouthparts with a corresponding forward protrusion of the head. The predators summarized this inter- Acron + 6 segments ganglion (SOG) pretation of insect of all insects head segmentation serves the major 1 1 - labral based on a careful ex- 2 mouthparts and 2 - antennal amination of develop- it’s therefore 4 3 - intercalary ing embryos of the much involved 5 4 - mandibular blister beetle Lytta. with feeding. stalkfly 6 5 - maxillary This picture is of The last three seg- a frontal section antlion larva 6 - labial ments are easy to see scorpion 3 through the SOG fly at all stages of devel- of a fly. acron opment (mandibular, neuropil maxillary and labial) because they bear ob- nerve labellar vious bi-lateral ap- tract nerve pendages. The three anterior segments required developmental analysis to confirm their pres- A typical ganglion (functional concentration of nerve cells) is composed of a periph- eral region of cell bodies (nerve and glial cells - red) and a region of nerve to nerve tsetse fly ence. The acron is homologous with the front part of the body of bi-laterally sym- cicada metrical animals such as the earthworm and is not considered a true segment connections called the neuropil. water 10 beetle 2 because it lacks a pair of appendages. larva 6 The three most prominent arrangements of the head have been given the names Prognathous, Hypognathous and Opisthorhynchous. This unusual view of the 1 suboesophageal ganglion (SOG) Plant bug of a fleshfly shows the brain and corpus pedunculatum SOG in the same orientation as archicerebrum the previous picture. Use the oe- 2 prosocerebrum sophagus as a reference point. Tiger 3 deutocerebrum 1 Here a single neuron has been beetle tritocerebrum 2 commisure filled with a fluorescent dye to re- brain labral nerve 3 veal its extensive branching pat- nervus connectivus tern in the SOG and in the brain. Prognathous 4 nervus recurrens commisure The cell body of this neuron is the frontal ganglion oesophagus bright dot just above the cell body nervus procurrens 5 4 label. This neuron is one of many 6 5 6 that process taste input from the labellar sensilla. Leaf Opisthorhynchous beetle The largest structure in the head-neck region is the brain and sub-oesophageal gan- suboesophageal glion complex. The oesophagus passes between these two major masses of nerv- cell ous tissue. These are dorsal and lateral views respectively. The numbers corre- body ganglion spond to the segmentation numbers of the previous slide. 11 7 Hypognathous 3 This frontal view of a cock- Dipteran mouthpart modifications roach head gives a better idea Mouthparts of Insects eye of what the mouthparts look labellum like in situ. The tentorium is REVIEW maxillary palp Insects owe their great success to their abilities to adapt to a antenna also visible (see module on the wide variety of habitats. Adaptations have occurred, indeed head in this series). Can you recognize the following? continue to occur, at every level of organization from the Note the well sclerotized man- molecular to the ecological. The comparative study of insect dibular “teeth” (black ridges in • antennae Horsefly adult- mouthparts is first and foremost a study of adaptation. We mandible lower center) and the protrud- Dissected ing galeae/lacinia and para- clypeus mouthparts single out mouthparts and the gross morphological level of • (ex. Micro-slide) maxillary organization in introductory courses because the variety of glossa. The mandibles appear palp to be at the front since the lab- • eye labrum adaptations is so great that we can address vast landscapes rum which largely covers them lacinia lacinia of insect evolution by looking at mouthparts alone. Also, they is transparent in this cleared • frons are relatively easy to see and appreciate and, perhaps most specimen. mandible hypopharynx mandible important, the mouthparts are entral to insect trophic • labial palpi behaviour cfood gathering behaviour). Horseflies (Tabanidae) have the same kinds of mouthpart modifications as labial maxillary • labrum blackflies, but since they are larger, are easier for us to study (their bite also causes more physical damage). All of the basic parts can be seen here and Supported in part by B.K.Mitchell and J.S.Scott palp palp mandible Academic Technologies for Learning © Department of Biological Sciences • don’t resemble the mouthparts of the cockroach. Nevertheless these are the and Faculty of Science, University of Alberta University of Alberta hypopharynx 4 8 appendages of the same segments - the result of the same basic develop- maxillary palpi mental plan. • 12 This detailed view of a Dipteran mouthpart modifications cockroach maxilla clearly cardo Mosquitoes have carried the business of piercing and shows the sclerites that The American cockroach sucking to a fine art. The stylets are modified labrum, make up this complex REVIEW (Periplaneta americana) has stipes maxillae and mandibles, just as in horseflies and appendage. The darker “generalized” mouthparts. Here’s a better challenge. This is a blackflies, but the cross section size is markedly re- the cuticle the heavier This insect is omnivorous and lateral view of a cockroach head with duced making for a much stealthier approach to feed- the tanning and usually its mouthparts are well suited the mouthparts extruded.
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