Methodological guidelines on tasks
For study it is desirable to use a rather large, freshly killed or alcoholic specimen.
The body is divided into three well-marked regions.
- The Head. Is it movable? Does it need to be as movable as your own head ? It bears several organs.
- The Thorax. Why should it be large and comparatively firm? This portion is more or less distinctly divided into three parts, each of which carries a pair of legs.
- The Abdomen. Count the number of segments. Each one is covered dorsally by a tergum and ventrally by a sternum. Why should the abdomen be more movable than the other por- tions? The posterior ends of the abdomens of male and female differ. This portion of the female is modified to form the ovipositor, which consists of two large pairs of plates that inclose a smaller pair of plates. It is between these plates that the oviduct opens. Why do the larger plates possess hard tips? Along the sides of the abdomen notice the stomata, the external openings of the respiratory system. Do you find stomata on other parts of the body?
(a) The compound eyes. Examine one with a lens or remove its outer covering and examine it with a compound microscope. You should understand the structure of the whole eye and how it gives a single visual image.
(б) The ocelli, three in number, one near the middle of the front part of the head and the others placed near the bases of the antennae.
(c) The antennce. Why are they so flexible? Examine one with a microscope and notice the spines. What are these for?
(d) Mouth parts. These should be studied later.
(a) Compare the three legs of one side. Do they have the same number of segments? Do all of the joints of the leg move in the same plane? The five divisions of a leg are, beginning with the basal end: coxa, trochanter (immovably joined to the coxa in the leaping legs), femur, tibia, and tarsus, which is com-posed of four movable pieces. Why do the femurs of the leap- ing legs differ from the femurs of the other legs? Determine how the foot is arranged to hold to objects. Have you noticed a grasshopper settle its feet preparatory to jumping? Examine the joint between the femur and tibia.
(б) Examine the wings and notice their size, shape, places of attachment, and general character. Do they apparently have different functions to perform? Notice how the posterior wings are folded so they may be covered by the anterior. Does this seem to greatly reduce their strength?
Draw an enlarged side view of a grasshopper, placing the appendages in their proper positions.
Mouth Parts. It has already been noticed that the mouth parts serve to cut off pieces of leaves, which are then passed directly into the alimentary canal. For such a purpose there should be holding as well as cutting parts.
- Pass a needle under the labrum, which forms the upper lip, and notice that it is hinged and that the end is lobed. It is You should examine the posterior wing of a beetle and see how it is folded. not supposed to be homologous with usual arthropod appendages. With fine scissors remove it and place it in a watch-glass containing water.
- Immediately behind the labrum is a pair of hard, dark-colored organs, the mandibles, that are used in cutting the food. Their position should be carefully noted, but it will be better to leave them in position until the other mouth appendages have been removed.
- Situated by the side of the mouth and just behind the mandibles are the maxillce. With a needle push one to one side and notice that it consists of a somewhat flattened portion with a jointed maxillary palp at one side. Carefully determine the positions of the maxillae with relation to other parts. What possible uses are served by the two parts? Remove them with scissors and place them in the watch-glass with the labrum, in approximately their relative positions and study carefully.
- Pass a needle behind the remaining appendage, the labium, and see that it is hinged and forms the lower lip. Remove it with scissors and place it in position in the watch-glass. You will find that it bears a pair of labial palpi, and that there is a deep cleft along the middle line. These are indications that the appendage is the result of the fusion of a pair of appendages.
- Remove the mandibles and examine their cutting margins. Place them in position in the watch-glass.
Make a drawing showing the structure of each of these appendages. Arrange your figures as nearly as possible in the relative positions of the parts.
Internal Structure. Remove the wings, and before opening the body notice the rather large, somewhat transparent tympanum on each side of the first abdominal segment, very near the base of the leaping leg. The structure of the auditory organ may be easily studied by staining, clearing, and mounting in balsam. (See Packard's " Text-Book of Entomology" or
The mouth parts of all insects that depend on biting off portions of plants for food are quite similar. Directions for the study of the mouth parts of the honey-bee are given furl her on, but the mouth parts of other forms, such as the fly, butterfly, and bug, should be studied.