Question

Characteristics of the class Arachnida include a. two tagmata. b. six walking legs. c. an aquatic lifestyle. d. a lobed body. e. both b and d.

Short answer

The correct answer is A. Arachnids have two tagmata.

Step-by-step solution

Examining Statement A

Class Arachnida, to which spiders, scorpions, and ticks belong, consists of two tagmata, the cephalothorax (head and thorax fused together) and the abdomen. So, statement A is correct.

Examining Statement B

Arachnids have four pairs of walking legs, not sex. So Statement B is incorrect.

Examining Statement C

Although some spiders and mites exhibit semi-aquatic lifestyles, Arachnids as a whole are not characterized by an aquatic lifestyle. Therefore, Statement C is incorrect.

Examining Statement D

A lobed body is not a characteristic of Arachnida. So Statement D is incorrect.

Examining Statement E

Since neither Statement B nor D is correct, statement E is also incorrect.

Key concepts

Arachnid Anatomy

Arachnids are fascinating creatures that are well-known for their unique body structure. Their anatomy is specialized for their predatory lifestyles. Structurally, arachnids are part of the Arthropoda phylum and therefore share some features with other arthropods, like having an exoskeleton and jointed appendages.

An arachnid's body consists of two main parts called tagmata: the cephalothorax and the abdomen. The cephalothorax is a vital body segment where the head and thorax are fused into one unit. This area holds the eyes, mouthparts, and a set of appendages called pedipalps which can serve various functions, as well as the chelicerae, pincer-like appendages for feeding. The second part, the abdomen, is behind the cephalothorax and contains the reproductive organs, digestive system, and the silk-spinning glands in spiders.

Understanding the arachnid anatomy is key for identifying different species and understanding their behavior, from the way they move to how they capture their prey.

Tagmata in Arthropods

Tagmata, the plural of 'tagma', refers to the specialized groupings of multiple segments into a cohesive functional unit in an arthropod's body. Arthropods, which include insects, crustaceans, and arachnids, have bodies that are segmented and often divided into tagmata. These serve as a prime example of form following function in the animal kingdom.

In arachnids, the two tagmata are the cephalothorax and the abdomen, which serve distinct and vital roles in the life of the organism. The division of the body into tagmata allows for specialization of different body regions. For instance, in insects, the typical tagmata are the head, thorax, and abdomen, each with specific functions like sensory processing, locomotion, and housing internal organs. The concept of tagmata is crucial in understanding the evolution, development, and functionality of these diverse and adaptable animals.

Arthropod Leg Count

One of the distinguishing features of arthropods is their legs – jointed appendages that are part of the exoskeleton structure. The number of legs is a characteristic often used to differentiate between the various classes of arthropods.

Arachnids typically have eight legs, which sets them apart from insects, that have six legs, and crustaceans typically with ten legs. These legs are mainly used for locomotion but can be adapted for other functions such as capturing prey, sensory perception, and even mating rituals. The number of legs is often an indication of the arachnid's life strategies. For example, spiders use their legs not only for moving but also for spinning silk and creating webs. Focusing on arthropod leg count is more than simple counting; it's about understanding the organism's interactions with its environment and how it has evolved to survive.