Phylogenetic Collection: Arthropoda
The house spider, the horseshoe crab, the cricket, and the housefly all belong to the phylem Arthropoda. These organisms share some basic characteristics, however, the differences between them point to differences in lifestyle, habitat, and food source. Anrthropoda is the largest phylem of the kingdom Animalia. Organisms of this phylem have segmented bodies, a hard exoskeleton, and jointed appendages, these appendages can have a wide range of functions. The exoskeleton of an arthropod is constructed of proteins and chitin in varying layers of thickness, depending on the organism (Campbell, 662). The exoskeleton serves two functions: as a layer of protection, and as a framework that the muscles attatch to, allowing the organism to move. Arthropods are fairly impermeable to water, but the hard outer covering doesn’t grow. Instead, they molt the exoskeleton as they grow. These organisms have a variety of sensory organs, an open circulatory system, and a tracheal system for respiration. The arthropod body structure means they are able to live in both aquatic and terrestrial environments, and they are found nearly everywhere in the world in all habitats. Arthropods range in size from the microscopic copepods constituting much of the ocean’s zooplankton to about 4m long- the leg span of the Japanese spider crab, currently the largest arthropod. Arthropods vary in diet, some are herbivores, others carnivores, still others omnivores. They reproduce sexually.
The house spider, horseshoe crab, cricket and housefly all have hard exoskeletons, segmented bodies, jointed appendages, and sensory organs. They have eyes and legs, though they vary in number. All of these organisms can move on land, molt during their lifetime, and reproduce sexually. Looking at the differences between these four organisms it may help to consider the further branching discussed in Campbell of four main evolutionary lineages: trilobites, chelicerates, uniramians, and crustaceans (663). Spiders and horseshoe crabs are chelicerates, while houseflies and crickets are unimarians.
Chelicerates have two main body segments, and most marine species are extinct (the horseshoe crab being one of the remaining marine species). They both have pincers, and book lungs (sometimes called book gills in the horseshoe crab), four pairs of walking legs, and are carnivores, feeding on other animals rather than plants. Horseshoe crabs are marine animals while spiders are terrestrial, so their lungs function differently. The horseshoe crabs’ lungs must always be moist, so they are limited in how much time they can spend on land. Some spiders can spin webs, and have a specialized system of glands to allow for this.
Unimarians have mandibles instead of pincers, antenna, and compound eyes. More specifically, these two unimarians are insects, so they have unbranched appendages (unlike crustaceans), and both of these specimens have wings. Flies have only one pair, while crickets have two. The cricket has biting and chewing mouthparts and undergoes incomplete metamorphosis, while the fly has sucking or lapping mouthparts and undergoes complete metamorphosis. It seems that complete metamorphosis allows for greater diversity as the fly order (Diptera) has four times as many species as the cricket (Orthoptera). These two live in similar habitats but are easily able to move to areas with more food.