Insect External Anatomy (Structure and function)

Transcription

1 Insect External Anatomy (Structure and function) I. The Arthropoda - insects (the class Hexapoda) belong to the phylum Arthropoda (jointed-legged animals). Arthropods arose in the pre-cambrian period (>600 mya). Arthropods are united by a number of important anatomical features (see phylogeny): 1. metameric segmentation 2. sclerotized cuticle 3. jointed appendages 4. tagmosis 5. compound eyes What makes insects so successful? Key morphological innovations of insects 1. metameric segmentation -- inherited from arthropod ancestors 2. exoskeleton composed of chitin (inherited from arthropod ancestors) with a waterproof wax layer 3. malpighian tubules and hind gut -- water conservation; crucial in allowing insects to colonize land 4. wings -- dispersal 5. mouthparts -- diet breadth 6. growth by metamorphosis 7. small body size -- biomechanical implications

2 II. Insect external anatomy A. Arthropod segmentation The insect body is subdivided into discrete, repeated units called segments. Each segment is characterized by a number of features (at least primitively): a. paired nerve ganglia b. jointed appendages c. sclerites, separated by regions of membrane d. coelomic spaces e. internal, sclerotized braces, called apodemes. Evolutionary/developmental implications of segmentation: a. segments are developmentally decoupled b. allows variation in the number of segments c. allows fusion of individual segments into composite structures, such as the insect head.

3 B. Arthropod cuticle The exoskeleton performs a number of extremely important functions: a) mechanical protection/armour b) skeleton -- site of muscle attachment/support and locomotion c) prevent water loss (wax layer) d) provides protection from the sun e) provides site for storage of waste products. The exoskeleton also has its drawbacks: a) limits growth of internal organs (e.g., ovaries, gut, etc.) b) requires periodic shedding for growth c) may be site of uptake of toxins -- especially fat-soluble toxins. 1. Structure of the integument a) basement membrane -- acellular layer b) epidermis -- one cell layer thick; produces overlying cuticle c) procuticle -- endocuticle + exocuticle (200 um thick) d) epicuticle -- thin layer overlaying the procuticle (only 1 um thick) but biologically very important. Procuticle Consists of two major components: a) chitin -- long chain polysaccharide (sugar) called N-acetylglucosamine. b) protein -- may be cross-linked by a process called tanning or sclerotization (caused by crosslinking of proteins with N-acetyldopamine and N-acetyldopamine quinone). exocuticle -- external; sclerotized and shed at each molt endocuticle -- internal; unsclerotized and resorbed at molting Epicuticle a) inner epicuticle -- tanned lipoprotein b) outer epicuticle -- tanned protein (1st layer produced after molting) c) wax layer -- primary protection against dehydration d) cement layer -- tanned lipoprotein How is cuticle both rigid and flexible? a) rigidity -- provided by tanning of the exoskeleton and by the presence of internal braces, called apodemes and apophyses. Individual regions of sclerotized cuticle are called sclerites; dorsal sclerites are called terga; ventral sclerites are called sterna. b) flexibility -- provided by membranous regions (called arthrodial membrane). Overlapping of sclerites provides both protection of the membranous regions and flexibility.

4 2. The molting cycle C. General external morphology The body in insects is divided up into three regions (or tagmata): head, thorax and abdomen. Each tagma performs a different function and has different organs located in it: head -- sensory system and information processing (brain); feeding thorax -- locomotion (legs and wings) abdomen -- digestive tract; reproductive organs; osmoregulation; blood circulation; mating; oviposition; sensory structures (cerci).

5 D. Abdomen -- Flexible overlapping terga and sterna; but abdomen is relatively derived in that walking legs are absent (except for most distal ones) segments total - segments 1-7 are pregenital - genital segments: 8-9 in females/9 in males - segments 10-11: often fused; segment 11 bears sensory structures called cerci. E. Thorax -- The thorax consists of three segments: prothorax mesothorax metathorax In primitive, wingless, insects the thorax retains a primitive configuration -- each segment is more or less similar in morphology and each bears a single pair of walking legs. The thoracic segments in flying insects are highly modified as a flight motor through fusion of the meso- and metathoracic segments and elaboration of the thoracic musculature. The flight motor is formed through the fusion of the meso- and metathoracic segments -- which bear the two pairs of wings. The prothorax plays no role in causing the wing movements. Insect legs -- The leg segments are: coxa trochanter femur tibia tarsus pretarsus

6 Insect wings -- Insects evolved flight in the Upper Carboniferous (~300 mya) while the earliest flying vertebrates, the pterodactyls, arose in the Triassic (~70 my later). Some of the earliest flying insects were huge dragonflies with wing spans approaching 1m. Insect wing stucture -- - thin wing membrane (composed of cuticle) held rigid between bracing elements called veins. Mechanisms of wing motion -- 1) dorsal longitudinal muscles -- cause wing depression; located on dorsal surface of the body 2) dorso-ventral indirect flight muscles -- extend dorso-ventrally; cause wing elevation by pulling notum (tergum) down with respect to sternum. Other, direct, flight muscles perform fine tuning of the wing s shape, camber, and angle of attack. Wing beat frequencies range from 10 Hz (large butterflies) to over 1000 Hz (tiny midges).

7 F. Head -- the insect head is a complex structure that has been formed through the fusion of multiple (6?) independent segments. What is the evidence for this? Remember the characteristics of arthropod segments: 1) nerve ganglia -- the insect brain consists of multiple (6) discrete nerve ganglia fused together. 2) Appendages -- insect mouthparts are clearly derived from highly modified legs. Head segments (at least according to one theory): 1) labrum -- not obviously an appendage, but because of neuronal connections to the brain, most likely primitively this was an appendage. 2) antennae -- annulated multisegmental appendages that provide sensory input 3) intercalary -- missing in adults 4) mandibles -- stout, hard, used in chewing 5) maxillae -- most similar to leg appendages; palpus = most of leg; used to manipulate food 6) labium -- basically two maxillae fused along the midline; provide sensory input and manipulate food.