ESPM 40 Insects and Human Society 10 Sept 14 WHAT ARE INSECTS? THE INSIDE Reading assignment. See bcourses for web links. The link to a web site covering Key questions: 1. Compared to humans, how are "insects are inside-out, upside-down, filled with air tubes, and fluid"? 2. What are advantages and limitations to the tracheal system of respiration in insects? 3. Why don't insects need to transport oxygen in their hemolymph? 4. How can females of some species continue to fertilize eggs for many days or even years after mating? Know terms and phrases underlined below. I. General internal organization - Key differences compared to vertebrates A. Skeleton is on the outside ("inside-out") B. Ventral nerve cord C. Tracheal system respiration D. Circulatory system for hemolymph (="blood") is an open circulatory system II. Prior to Marcello Malpighi (1669) it was generally thought insects did not have inter-nal organs. III. Alimentary tract (“gut”) A. Organization 1. tube open at both ends: enter at one end, exits the other (just like us!) 2. compartmentalized for ingestion, digestion, excretion B. Foregut = an invagination of the outside of the insect 1. lined with water-proof cuticle - transports and crushes food; some di-gestive enzymes or feeding aids (e.g.: local anesthetics for blood feeders) may be added in saliva 2. may store or crush food 3. know oesophagus, crop and proventriculus C. Midgut = main digestive and absorptive portion of gut D. Malpighian tubules = excretory organ (analogous to mammalian kidney) 1. maintains ion (salts) balance of hemolymph 2. eliminates breakdown products (waste), the most toxic of which are usually nitrogen 3. empties into beginning of the hindgut E. Hindgut = storage/passage of digested food and excrement; some absorption, especially water IV. Nervous system ("upside-down") A. Organization = series of segmental ganglia 1. ganglia interconnect nerves to sensory organs and to muscles 2. insect brain consists of several groups of large ganglia in the head B. Basic neural circuitry similar to other animals, including vertebrates1. sensory organs trigger nerve impulses (waves of electrical depolariza-tion of nerve membrane) 2. nerve signals transmitted chemically across the gaps (synapses) between nerve cells. Some insecticides act by interfering with this transmission. V. Tracheal system (full of air tubes) - a key to insect success on land A. Spiracles on exterior open/close to admit air exchange with outside 1. preserves water by limiting time that interior of tracheal system is open to outside air B. Tracheae are highly branched 1. tracheae are cuticle-lined and impervious to water except at extremity (limits water loss) 2. no cell is more than a couple of cells away from a tracheole, where oxygen and carbon dioxide exchange in tissues occurs 3. larger parts of tracheal system can act like bellows to move air in & out more rapidly, but most gas movement is by simple diffusion. Because of small size of insects, this is fast enough to meet their needs. Diffusion is not fast enough for large animals. Why? C. Aquatic insects 1. may have spiracular ventilation just like terrestrial insects - gas ex-change between air and tracheae (example: mosquito larvae) 2. some have tracheal gills (closed tracheal system) - gas exchange be-tween air in tracheae and air dissolved in surrounding water 3. others exchange gases through the exoskeleton VI. Circulatory system ("filled with ... fluid") A. Open circulation system - body cavity is a hemocoel (primary body cavity) 1. role of hemolymph is typically NOT gas exchange (rare exceptions) 2. It does transport nutrients, hormones, heat, defensive cells, etc. B. Movement of hemolymph is much more sluggish than animals with closed cir-culatory system. No true blood vessels except for the dorsal vessel often referred to as the "heart" or "aorta". VII. Reproduction A. Female reproductive tract 1. stores sperm in a sac (spermatheca) off to one side of the genital cham-ber a. mated females may continue to fertilize eggs for a long time without further mating 2. some insect species reproduce without mating (parthenogenesis) during some phase of their life 3. females of some species give live birth (relatively rare) 4. know ovaries, spermatheca, accessory glands B. Male role in reproduction: mating 1. after mating, male role in reproduction is usually completed 2. produce sperm in packets (spermatophore) or in a discharge of seminal