Download Arthropods: compare crayfish and grasshopper

Arthropods
The animal Phylum Arthropoda includes many, many species, who share a few characteristics:
their body / muscle support is on the outside, in the form of an exoskeleton which covers their
whole bodies, and is jointed at flexing points; they generally have many pairs of legs; they
circulate blood through their hollow interior, through what is called an open circulatory
system; they, like many other invertebrate groups, have a nervous system whose central cord
is paired and solid and runs along their ventral (belly) surface. They include arachnids, like
spiders, scorpions, ticks, and mites, and two groups we'll look at in the lab - the crustaceans,
mostly aquatic, and the insects, which are mostly terrestrial and usually winged.
Part One: Crayfish
Crayfish are common freshwater crustaceans, related to lobsters, crabs, and shrimp. They are
bottom dwellers and live as predators and scavengers. They live in small hollows or burrows
that they back into, with their chelipeds, or claws, facing out.
External Anatomy
Get your dissection equipment, including a dissection pan, and a preserved crayfish. Go online
and find a labeled diagram of a crayfish to help identify structures. Crayfish, like all
crustaceans, are also characterized by doubled appendages - note that, although the main
antennae are single, the shorter antennules below them are doubled; we'll also later see
doubled structures at the bases of the legs.
1. The front legs, the chelipeds, are supposed to be a specialized adaptation of one of the sets of
regular walking legs - this means that the chelipeds and the walking legs are homologous, built
from the same basic structure. Look closely at them - list similarities that support this idea?
2. The cheliped, of course, is not a walking leg - it's adapted for defense and for grabbing prey.
How is it different from a walking leg?
3. Arthropods are considered to be segmented - their basic body plan, at least as embryos,
consists of repeating units. Some of this feature remains visible on the outside. What parts of
the crayfish body show signs of segmentation?
4. What parts of the crayfish do not seem to be segmented?
Inspect the swimmerets - they are the fine, leglike (they are also modified legs) structures
under the abdomen, behind the walking legs. In females, they all look roughly the same, but
males have much larger anterior ones which are used to transfer sperm packets from the male's
genital opening (a small pipelike structure just in front of the swimmerets, usually difficult to
see) to the female's. The female receives sperm in the autumn and stores it in a seminal
receptacle until the following spring - she then lays her eggs and deposits the sperm on them.
The fertilized eggs (and the larvae that hatch from them) are then carried around attached to the
mother's swimmerets for as long as three months.
5. Which gender is your crayfish specimen? ______________
6. Is fertilization in crayfish external or internal? Explain briefly.
Carefully pry up the carapace, the shield-like covering, and cut it away from the crayfish. This
should expose the gills. When out of the water, a crayfish can continue to breathe as long as it
can keep some water trapped in there around the gills.
7. How many sets of gills are there on one side?
Take a walking leg by the base and twist it so that both it and a gill are detached. In a petri dish,
float the gill. Note how, supported by the water, the gill spreads and reveals its complexity.
8. Why is the gill so intricate?
Arthropods - Part Two: Grasshoppers
Grasshoppers are insects, the largest arthropod subgroup. There are more named insect species
than in any other group of animals; whether there are more actual species is much more
difficult to determine. Insects are considered a terrestrial, or land, group. Grasshoppers are a
type of herbivorous (plant-eating) insect from an order that includes crickets, mantises, and
cockroaches. Our specimens are from a particularly large species of grasshoppers, commonly
called "lubbers."
External Anatomy. Get a preserved grasshopper specimen. The bodies of insects are divided
into three sections: the head, with many of the sensory organs, the main processors / brain, and
the mouth; the thorax, with its six legs (insects characteristically have six and only six true
legs) and, in most adults, four wings (although wings have often been adapted as protective
covers); and the abdomen, which usually carries the sex organs. Go online and find a labeled
diagram of the grasshopper for reference.
1. Compare. How are the grasshopper's external features similar to those of the crayfish?
2. Contrast. How are the grasshopper's external features different from those of the crayfish?
The Head
Locate the large compound eyes. Arthropod eyes typically don't have one big lens in front of a
screen of detectors, like our eyes - they have a layer of small lenses, each with a small set of
detectors. This takes up much less space. Grasshoppers do have three single-lens eyes, above
and between the bases of the antennae. The antennae are used for other senses, such as touch
and some parts of the sense of smell. Take a scalpel and carefully make a small shaving from
the surface of the eye. Mount the slice on a microscope slide (dry) and look at it at low and/or
medium power.
3. Describe and sketch the appearance of your slice of compound eye.
Around the mouth opening are a number of different types of mouthparts. Like a crayfish's
chelipeds (and some crayfish mouthparts), several of these are modified legs. Note the
somewhat leglike palps that help move food around, the plates that provide a kind of "cheek"
cavity, and the tongue-like hypoglottis and grabbing and chewing mandibles in the actual
mouth opening.
The Thorax
The thorax is derived from three original segments, each with a pair of legs; the last two, in
many forms (but virtually always only in the adult stage) also support wings. The thorax is
usually very solidly built, to support the power of the walking and flying muscles. In many
insects, the grasshopper included, the front set of wings may be very different from the back
set. Compare and contrast the wings in your grasshopper - be sure to spread the back wings.
If your specimen is not an adult, the wings are not developed and may be small and difficult to
do this with.
4. The front wings:
5. The back wings:
The Abdomen
On the first abdominal segment, just behind and below each wing, is a structure called the
tympanum. Humans have a similar structure with the same name and function (but it's not on
our abdomens).
6. Describe the grasshopper's tympanum.
7. The tympanum contains many nerves and processors. It picks up vibrations carried in the air.
Why?
Grasshoppers, as most insects do, breathe through a tracheal system, an organization of tubes
that connect the cells of the body directly with the outside air. The blood carries almost no
oxygen. The openings for the system, spiracles, can be seen along the sides of the thorax and
abdomen. Also, the reproductive structures are on (and in) the abdomen. Female insects can
often be recognized by their ovipositors - structures specialized for laying eggs. Since
grasshoppers dig a shallow hole for a packet of eggs, the ovipositor is a large spade / pusher,
and only seen on the female.
8. Is your grasshopper male or female?
WRAP UP
9. What are some common features of ALL Arthropods?
10. What is molting? Why do Arthropods molt?