Video Transcript
Which feature is used to classify
arthropods into smaller groups? (A) Types of teeth, (B) number of
teeth, (C) number of legs, (D) number of wings, or (E) type of reproduction.
This question is asking us what
feature can be used to identify the arthropod subgroup an animal fits into. To answer this question, let’s look
at classification in general and then the arthropods in more detail.
Classification is the arrangement
of organisms into taxonomic groups according to their observed similarities. This biological classification is
based on a hierarchical system, where you start with large groups and then split
them down into smaller and smaller groups based on their features. Here we are starting with a kingdom
— animals and plants are both kingdoms — and finishing with species. The animal kingdom, for example,
can be divided into vertebrates and invertebrates.
Vertebrates are animals with a
backbone, and, as you may guess, invertebrates are animals without a backbone. We are vertebrates, and we have an
internal skeleton called an endoskeleton. Endo- means internal.
The largest group of invertebrates
are the arthropods. This is the group of animals
mentioned in this question. An arthropod is an invertebrate
animal with an exoskeleton, segmented body, and jointed appendages, bits that stick
out like legs and antenna. An exoskeleton is a rigid exterior
covering for support and protection. These arthropods can then be
further arranged into subgroups by, again, looking for similar features. Let’s now look at a few of these
features.
Insects have a body divided into
three segments called the head, thorax, and abdomen. They have six legs in three
pairs. The legs and usually one or two
pairs of wings are attached to the thorax. They feed in many different
ways. For example, ants use their jaws to
break their food up and bees use a proboscis, a type of feeding tube, to suck up
nectar from flowers. Most insects reproduce by laying
eggs.
Arachnids, which include spiders
and scorpions, have a body divided into two. Their head and thorax are merged
together to form the cephalothorax. They have eight legs, four pairs
attached to the cephalothorax. Arachnids do not have wings or
antenna, and they only eat liquid food as they have no teeth to chew. They usually lay eggs except for
scorpions, which give birth to live young.
Myriapods have many segments to
their body, and most segments have a pair of legs attached. This means they can have lots and
lots of legs. The name myriapod actually means
many feet. Centipedes and millipedes are
examples of myriapods. They have modified appendages on
their head to grasp and bite food, but they do not have teeth. Myriapods have antennae but no
wings, and they reproduce by laying eggs.
Having reviewed some key points
about different subgroups of arthropods, let’s return to the question and go through
the answer options.
No arthropods have teeth, so we can
rule out options (A) and (B).
Insects can have one or two pairs
of wings but still belong to the same subgroup, so we can rule out option (D).
Arachnids can either lay eggs or
give birth to live young, so we can rule out option (E).
However, if you look at insects,
they have six legs, arachnids have eight legs, and myriapods have many legs. Therefore, the correct answer to
this question is (C). The feature used to classify
arthropods into smaller groups is the number of legs.
