WEEK 11 HANDOUT FOR BIOLOGY 03050, INVERTEBRATE ZOOLOGY 11/8/99
ARTHROPODS II: INSECTS AND MYRIAPODS
Text (7th ed.): Ch. 23, pp. 508-530
Dr. W. Crone (303 FTZ, 629-7439, cronewil@hvcc.edu, http://www.hvcc.edu/academ/faculty/crone/index.html)
possible web site: http://www.ent.iastate.edu/List/ (links to insect images, sounds, etc.)
possible HVCC a/v source: VT 3380,"Eyewitness: Insect" (to be seen in lab this week), and VT 1180,"Life on Earth: Swarming Hordes" (societal insects)
THIRD LECTURE EXAM ON TUESDAY, 11/16/99, COVERS THROUGH THIS WEEK
(also note that next Friday, 11/19/99, is the last day to withdraw from classes)
1. Phylum Arthropoda, Subphylum Uniramia, Class Insecta: There are more species of insects than any other organisms on earth. Major reasons for this success include the following:2,3
a. exoskeleton (the protective cuticle which can be modified, e.g., camouflage)
b. ability to fly (certainly opens up new possible niches and habitats)
c. larval stages (less competition between parent and offspring)
d. small size (think of how many more places an insect can live vs. a larger organism)
e. high fertility (e.g., 10,000 eggs/day by a termite queen)
2. Main external features of insects include:
a. three tagmata or body regions of head, thorax, and abdomen. Tagmatization is consistent and visible among insects.
b. mandibles as main mouthpieces on head. While mandibles are primitive and chewing, insects may have highly modified mouthparts, e.g., sponge-like mouth of housefly.
c. both compound eyes and simple eyes on head. Compound eyes do most of the"seeing," but additional simple eyes seem to assist with interpreting light intensity.1
d. three pairs of legs (vs. four pairs for arachnids and chelicerates) off of thorax. As with other arthropods, these limbs may become highly modified by evolutionary selective pressure, e.g., the gripping forelimbs of a praying mantis vs. the jumping hind limbs of a grasshopper.
e. wings (usually 2 pairs) off of thorax. Wings are modified cuticle, so are analogous to bird wings, but not homologous. Veins in the wings represent where blood supply, etc., once existed.
f. no appendages off of abdomen (unlike the swimmerets in the crayfish you dissected), but will see spiracles (tracheary openings) along the length of an abdomen.
3. Internally, an insect will have many general similarities to other arthropods. Two examples of internal insect features of interest for the insect lifestyle on land:
filtration of liquid wastes: insect blood is in an open circulatory pattern where it bathes internal organs. As a result, the functioning kidney structure or Malpighian tubules surround the gut and dump the urine equivalent into there (think of the"tobacco juice" of a grasshopper therefore being a mix of feces and urine), helping to conserve water.
respiration: tracheae deliver air directly to muscles (an insect will want to stay aerobic while flying, right?), where movement in the abdomen can help to"pump" and encourage air circulation.2
4. Flight offers major benefits to insects. There are two major patterns in insects:
a. direct flight muscles, with attached muscles down on wing base--seeen in dragonflies.
b. indirect flight muscles, with the downstroke from muscles not attached to the wings, e.g., flies.
Indirect flight muscles contracting on the exoskeleton account for the upstroke. In either case, the wing angle keeps changing to account for change in directions.2
5. Life cycles are important in insects: incomplete and complete metamorphosis.
(few): direct development (silverfish): egg-(juvenile)-adult
(<10%): incomplete metamorphosis (grasshopper), grows up without a pupal or resting stage: egg-nymph-adult
(90%): complete metamorphosis with pupa, with its large rearrangement of internal organs into adult: egg-larva-pupa-adult
6. Let's highlight the four largest orders of insects:1,2
a. Coleoptera ("sheathed wing"): beetles: 300,000 species (vs. only 45,000 vertebrates!).
Why so many beetles?
1) Complete metamorphosis, progressing from grub to winged form. These stages live in different areas and eat different foods.
2) The front pair of the beetles' two sets of wings are thickened into hard covers. These fold back into an effective shield, protecting the soft body underneath, hence the order name.
3) Beetles have kept their primitive mouth parts, designed for chewing abundant solid foods. Many other orders have mouths fit only for more specialized meals, e.g., sipping sap.
b. Lepidoptera ("scaly winged"): butterflies and moths: 140,000 species. Tiny shingle-like scales cover the wings and bodies of most. Main differences between butterflies and moths:1,2,4
1) Moths spin cacoons, butterflies spin a chrysalis.
2) At rest, moth tends to fold its wings like a tent, while the butterfly presses them together overhead.
3) Moths are more subtly colored and fatter bodied than butterflies.
4) Moths tend to be more nocturnal than butterflies.
c. Hymenoptera ("membrane winged"): bees, wasps, and ants: 120,000 species. Note the familiar wasp waist, where one segment of the abdomen is pinched in. Also the thin, transparent wings, hence the order name.1,2
Useful for us: pollinate, turn soil over (think of ants), even directly give us food like honey. Most importantly, though, they (especially wasps) prey on other insects and are the single most important factor in keeping overall insect population in check.
d. Diptera ("two winged"): flies, gnats, and mosquitoes: 90,000 species.1,2
A lot of our worst enemies here. Most have a single pair of wings, hence order name (the hindwings are modified into halteres that help with flight balance). Many also have tubular mouths for sucking and can transmit diseases, e.g., malaria, sleeping sickness, filariasis, yellow fever. A house fly does not break the skin, but will land on food, vomit up part of its last meal and can transmit diseases as well, e.g., typhoid.
7. Three other common orders to be aware of:2
a. Odonata ("toothlike")--dragonflies and damselflies. Aquatic nymphs, four membranous wings with crossveins, very short antennae, large compound eyes for hunting.
b. Orthoptera ("straight winged")--grasshoppers, crickets, praying mantids, and cockroaches. Thickened forewings with veins that cover hindwings, with chewing mouthparts.
c. Hemiptera ("half wing")--true bugs, e.g., stink bugs, milkweed bugs. Anteriors of front wings leathery with posteriors membranous, hind wings fully membranous, piercing and sucking mouthpieces.
8. Comparison of insects vs. myriapods: especially different in number of pairs of legs (3 pairs in insects vs. many in millipedes and centipedes) and tagmatization (head, thorax, abdomen in insects vs. head, trunk in millipedes and centipedes). Also, without much wax in the outer covering (epicuticle), myriapods are restricted to moister settings than insects.3
9. Subphylum Uniramia, Class Diplopoda ("double foot"): millipedes. Millipedes live under rocks, etc. eating decaying plant matter. Rounded in cross section; two pairs of legs per visible segment, with"low-gear" gait.
10. Subphylum Uniramia, Class Chilopoda ("lip foot"): centipedes. Centipedes with one pair of legs per visible segment. Dorsoventrally flattened in cross section; fast-moving predators with"high-gear" gait.
1 P Farb, The Insects (Time-Life Books, NY, 1962), pp. 22-29, 36.
2 CP Hickman et al., Biology of Animals, 7th ed. (WCB McGraw-Hill, Boston, 1998), pp. 509, 511-513, 522-523.
3 NM Jessop, Schaum's Outline of Theory and Problems of Zoology (McGraw-Hill, NY, 1988), pp. 223, 226.
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This page updated on November 8, 1999