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
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.
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.
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).
D. Abdomen -- Flexible overlapping terga and sterna; but abdomen is relatively derived in that walking legs are absent (except for most distal ones). - 11 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
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).
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.