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Phylum Arthropoda
(CCanadian Campbell 2nd ed Pages 746 - 753)
This group of organisms constitutes the most diverse in term of numbers of species of all plant and animal phyla. In fact, there are more species of arthropods on earth than all other organisms combined. Recent evidences suggests that there may be as many as 30 million different species in this group alone. Arthropods are thus considered to have attained the greatest "biological success". They occupy the greatest variety of habitats, consume the largest amounts and kinds of foods and are capable of defending themselves. This phylum also shows a tremendous diversity of form and of size, the smallest is a mite measuring <1 mm in length and the largest, the Japanese Spider Crab, is 4 meters or 13 feet in size.
Arthropods are divided into 9 major classes with living representatives. For our purposes, we will focus on the classes Crustacea, Insecta and Arachnida.. The chitinous exoskeleton, jointed appendages, highly integrated nervous system and haemocoel are among the main characteristics of this group. They are believed to have evolved from an ancestor of the polychaete annelids.
Grasshoppers belong to the Order Orthoptera, and we will be studying them as representative insects. Grasshoppers live all over the world, mainly in grasslands, weed patches and roadside growth, where they eat grasses and other vegetation.
The
body of the grasshopper shows segmentation but many of the segments (somites)
are fused. The head is made up of six fused somites; the thorax
of three somites to which are attached legs and wings; and the abdomen
is made up of many segments. The head has one pair of slender jointed antennae,
two compound eyes and three simple eyes or ocelli. The mouth
parts are of the chewing type and include a broad upper lip or labrum,
two heavy blackish lateral jaws or mandibles,
a pair of maxilla
made of several parts, and a broad lower jaw or labium. The slender
abdomen
consists of many somites, the terminal ones being modified for reproduction.
Along the lower sides of the thorax
and abdomen are 10 pairs of small openings, the spiracles.
These connect to a system of tubes or tracheae
which are used for gas exchange.
Examine the grasshopper externally:
Draw a sketch or print the image and label.
Mouth structures of insects vary. Give examples of three different insects each having a different mode of obtaining food, i.e. chewing, piercing and sucking, and lapping.
Examine the dissected grasshopper:
Observe the following: crop, gastric caeca, gonads, stomach, Malpighian tubule, intestine, rectum, anus. Make a sketch of the grasshopper showing these internal features.
Among the insects, two types of life cycle development are known, complete metamorphosis and incomplete metamorphosis. The Grasshopper exhibits incomplete metamorphosis.
Click here to view examples of metamorphosis.
Some of the other members of this class include the Butterflies & Moths (O. Lepidoptera), True Flies (O. Diptera), Dragon- and Damselflies (O. Odonata), Beetles (O. Coleoptera) and the Ants, Bees and Wasps (O. Hymenoptera).
