Laboratory Materials

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Topic 7.  Escape behavior.

________________

(Name)    

I. MATERIALS.

            Adult or late instar American cockroaches, Periplaneta americana.

            Adult cyclorrhaphan fly, housefly or larger.

            Super glue.

            Dental floss.

            Dissecting scissors.

            Stop watch.

            A small wooden stick or other convenient object to attach to the thorax to mount the adults.  The same mounting technique used in the proboscis responses can be used here.

            A convenient substrate.  For the flies, this can be a bit of paper or Styrofoam.  Grasshoppers may be left on the table top or in a large container. 

            A large globe, or the equivalent on which to mount the cockroach with  compass directions indicated to document plot movement.

II. LEARNING OBJECTIVES.

            Each student should be able to:

            Describe the insect behaviors triggered by various sensory inputs.

How:   

            Observe the aborted escape responses evoked in headless cockroaches.

            Observe the start to flight response of the adult fly.

            Observe the maintenance of flight by stimulation of cephalic hairs.

III. INTRODUCTION.

            Escape responses evolved in insects to avoid capture by predators.  Perhaps the best known are the wild flight maneuvers evoked in adult moths in response to ultrahigh sound signals that duplicate echo-locating sonar of bats, and popularized by Kenneth Roeder who worked at Tufts University in Massachusetts.  Because these escape behaviors have evolved under intense selection pressure between prey and predator, they are characterized by short response times and very simple nervous pathways between sensory detection and motor response.

            There are only a few ways to shorten response times in the nervous system.  Large diameter axons can be selected for in evolution because conduct velocity of nervous impulses is directly dependent on the square root of axon radius, the larger the radius, the faster the conduction velocity.   Time can also be saved by selecting the shortest route possible with a fewest nervous elements employed.

Cyclorrhaphan Diptera escape response.

            All of these strategies appear to have been selected for in producing the start-to-flight reflex response of modern cyclorrhaphan Diptera.  "Giant" interneurons gather information from optical ganglia and send it directly to the motor neurons of large tergotrochanteral jumping muscles in the mesothorax. The giant interneuron appear to make electrical connections with the motor neuron, eliminating synaptic delay in the neighborhood of a millisecond. 

            This advantage in short-time response is why adult houseflies are so adept at avoiding the fly swatter.  The reflex response is so fast, it has to be captured on film and replayed in slow motion to be observed.  The first thing an escaping fly does in response to approaching shadows is depress the trochanter which extends both middle legs.  Since this is a paired nervous pathway, it affords the fly the option of jumping to either side.  The second event in escape, unfolding of the wings, begins even before the legs are fully extended. 

            The implied connection between tarsal contact and the flight apparatus is evident in the so-called start-to-flight response of flies.  As soon as the tarsi leave the substrate, the flight motor apparatus begins moving the wings as a reflex.  And in reverse, when the legs touch down on a surface.  This reflex response can be seen by alternately pulling the fly free of a substrate and returning it. 

The cockroach escape response.

            The escape response of the American cockroach using the cercal nerve-giant axon pathway is, if anything, even more studied than the flight responses of the fly.  The cerci are conspicuous appendages at the tip of the abdomen.  They are covered with trichoid sensory hairs which act as wind-sensitive mechanoreceptors.  When stimulated by air puffs, these hairs deliver a barrage of nervous impulses that ascend into the terminal abdominal ganglion (TAG).  Here the sensory axons synapse in the neuropile with seven very large interneurons, that send large diameter (termed giants, by some) axons forward through the middle of the ventral connectives.  The giants are categorized by the position of their axons in the connectives.  There are 4 ventral giants and 3 dorsal giants in each connective.  Each of these giant axons correspond to one large neuron cell body located on the contralateral side of the TAG to the connective which contains its axon.

            Three of the ventral giants have the fastest conduction velocities and are the first to respond to air puffs on the cerci, but are strongly inhibited during walking behavior (Collin, 1985).  The dorsal giants are initially inhibited during escape responses and are mildly excited during walking.  Thus only the ventral giants are thought to play a direct role in initiation of escape behavior.

            The possible role of these giant interneurons in ordinary behavior on the cockroach is little understood beyond the escape response.  All giant axons traverse the entire ventral nerve cord and eventually project into the suboesophageal ganglion, not the brain.  They send ramifying branches into each of the ganglia they traverse in their pathway from the TAG to the suboesophageal ganglion.  The ramifications in the thoracic ganglia are greater that those in any of the abdominal ganglia.

            Cobalt impregnated axon fills show that the cell bodies of the giants lie internally against the side of the TAG, send a single process across to the neuropile on the opposite side of the TAG (contralateral) where a synaptic arborization is formed, then a single axonal process projects forward from the neuropile into the connective contralateral to the cell body.  Thus sensory information coming from the left cercus is transferred to an interneuron running up the ipsilateral connective, via synaptic connections on the ipsilateral (same) side of the TAG, but whose cell body on the opposite or contralateral side of the TAG. 

IV. DIRECTIONS.

            Remove a large cockroach from culture.  Anesthetize with cold or carbon dioxide gas.  Tie a piece of dental floss around the neck and draw it tight.  Trim the ends of the floss so they are out of the way.  Decapitate distal to the floss leaving the body intact.  Crust and discard the head.  If you did not use disposable gloves, wash your hands.

            Wait for the effects of the anesthesia to wear off, about 1-3 minutes, depending on the length of exposure to anesthetic conditions.  The freshly prepared cockroach might appear skittish at first, with jerky movements; however, in a few minutes, it will settle down. 

            When placed on a substrate, ether vertical or horizontal, the recovered cockroach will maintain a posture.  Carefully note the nature of the posture, the height of the body off the substrate (if on a horizontal surface), and the position of the legs.

            Is the abdomen held off the horizontal substrate?  If not describe what part touches the substrate.

            _________________________________________________________________.

            In full view of onlookers, the TA should take a disposable pipette with a large rubber bulb attached and direct a puff of air at the cerci on the rear of the abdomen.  The cockroach will lurch forward a few steps, then stop.  If this process is repeated frequently, the cockroach will no longer respond to the air puffs.  Lack of response to continued stimulation may be termed habituation to the stimulus. 

            TA might do this by placing the cockroach in the middle of a circular compass field similar to the one indicated in the workbook (below).

            The TA should also be able to demonstrate that a puff directed at the left rear cerci will produce a directed lurch to the right or away from the stimulus.  Repeat on the right side.  Produce a direction diagram of the position of the stimulus and direction of the resulting movement.  (It may be most convenient to place the cockroach on a large globe that can be centered in the middle of a surrounding compass field before each stimulus is delivered.

            After habituation has been demonstrated, start a stop watch, and measure how many seconds it takes to regain full response to the  air puff stimulus.  Score the response on the time line below:

   Last stimulus delivered

                      ¯

Time:              0    5   10  15  20  25  30  35  40  45  50  55  60  65  70  80  90  100

Response:      __  __  __  __  __  __  __  __  __  __  __  __  __  __  __  __  __  __

            Use a plus sign or minus sign to indicate response.  Be sure to measure the entire time from the last puff to the test stimulus.  A test puff will reset the habituation clock. 

Housefly lost of tarsal contact.

            Mount an adult fly on a stick probe in such a manner that the wings are free to move (similar to the proboscis response experiment).  Position the fly over a substrate so that the legs are in contact, and the fly assumes a posture.  Now yank the fly off of the substrate suddenly so that flight is initiated.  How long does the flight last (about)? _____________.

            Notice while the fly is in flight, gently blowing on the head, simulating the flow of air over the body, reinforces flight and prevents the fly from stopping its wings while suspended.  Notice the positions of the legs during flight.  Draw these on the accompanying diagram.

            Notice the position of the legs when you suspend the adult fly and it stops flying.  Are they different than the flight position?  ___________.

            bring a substrate close to the suspended fly with its legs in the flight attitude.  What happens when the substrate comes in contact with the front legs of the suspended fly?

            _________________________________________________________________.

V. SUMMARY ANALYSIS.

            Describe what was learned from this exercise.

VI. REFERENCES.

 Collin, S. P. 1985. The central morphology of the giant interneurons and their spatial relationship with the thoracic motorneurons in the cockroach, Periplaneta americana.  J. Neurobiol. 16: 249-267.

Zill, S. N. and Moran, D. T. 1973.  Suppression of reflex postural tonus: a role of peripheral inhibition in insects.  Science.  216: 751-753.

_________________

(Name)                       

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