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7. SEXUAL REPRODUCTION

Sexual reproduction or the uniting of female and male gametes is the usual case in arthropods. However, some insects are parthenogenic, females produce female offspring without mating. In the Hymenoptera (bees and wasps) Haplodiploidy (Haplodiploidy is a sex-determination system in which males develop from unfertilized eggs and are haploid, and females develop from fertilized eggs and are diploid) is common. An example is the honey bee where the queen bee stores sperm and selectively fertilizes her eggs, unfertilized eggs develop into males and fertilized eggs develop into females



Classification of the Phylum Arthropoda:

Phylum Arthropoda (Arthropods) is the largest and most successful of the animal phyla. All arthropods have segmented bodies divided into a head, jointed legs and abdomen.

The major Classes of living Arthropods are as follows with one example each:


(01) Class Arachnida - spiders
(02) Class Crustacea - crabs

(03) Class Merostoma- horseshoe crabs


(04) Class Diplopoda -  millipedes
(05) Class Chilopoda - centipede
(06) Class Insecta or Hexopodainsects
(07) Class trilobites are an extinct group of arthropods that lived in the seas of the world for about 380 Mya (million years ago), from the Precambrian 610 Mya to around the end of the Permian 230 Mya.

(01) Class Arachnida - spiders


Arachnids (class Arachnida) form the second-largest group of land arthropods (phylum Arthropoda) after the class Insecta. There are over 70,000 species of arachnids that include such familiar creatures as scorpions, spiders, harvestmen or daddy longlegs, and ticks and mites, as well as the less common whip scorpions, pseudoscorpions, and sun spiders. The marine horseshoe crabs and sea spiders are near relatives.



Physical characteristics:

(1) Arachnids have paired, jointed appendages (parts that are attached to the main body), a hardened exoskeleton (exo means "outer"), a segmented (divided into parts) body, and a well-developed head.

(2) Their body consists of two main parts: a fused head and thorax, and an abdomen.

(3) There are six pairs of appendages on the body: the first pair are clawlike fangs near the mouth used for grasping and cutting; the second pair serve as general-purpose mouth parts that may be modified for special functions; and the last four pairs of appendages are the walking legs.

(4) Most arachnids live on land and breathe by means of book lungs (so called because their thin membranes are arranged like the pages of a book) or by tracheae (small tubes that distribute air from the outside throughout the body), or both.

(5) Most are flesh-eating predators. They feed by piercing the body of their prey and directly consuming its body fluids or by releasing digestive secretions that predigest the food before they eat it.


Salient Features of some special types of Arachnids:


  1. Scorpions

(1) Scorpions have large, pincer like second appendages and a segmented abdomen that is broad in front and narrows to become tail like, ending in a sharp pointed stinger.

(2) The stinger contains a pair of poison glands with openings at the tip. The venom is neurotoxic (poisonous to the nerves) but, except in a few species, is not potent enough to harm humans.

(3) Scorpions have book lungs for breathing.

(4) They engage in complex courtship behavior before mating, and newly

hatched young are carried on the mother's back for one to two weeks.



(5) Scorpions are nocturnal (active at night) and feed mostly on insects. (6) During the day they hide in crevices, under bark, or in other secluded places. They occur worldwide in tropical and subtropical regions.


  1. Spiders

(1) In spiders, the abdomen is separated from the joined head and thorax by a narrow waist (‡KvwU ev †Kvgi eÜbx).

(2) The large and powerful first appendages of some spiders contain poison glands at their base, while the tips serve as fangs (wel `uvZ) that inject the poison into prey.

(3) The second appendages of spiders are long and leg like. In male spiders, these appendages each contain an organ used to transfer sperm to the female.

(4) Some species of spiders have only book lungs for breathing, while others have both book lungs and tracheae.

(5) Spiders have organs called spinnerets at the tip of the abdomen that contain silk glands. The spinnerets draw secretions from the silk glands to produce fine threads of silk. These are used to build webs, ensnare prey, package sperm to be transferred to the female, and make egg sacs.

(6) Although all spiders produce silk, not all weave webs. Spiders have courtship patterns prior to mating that are quite varied.

(7) Spiders are found worldwide and live in many different habitats—in burrows in the ground, in forests, in human habitations, and even under water.

(8) Spiders are predators, feeding mostly on insects. Despite their reputation as fearsome(fq¼i) animals, they actually benefit humans by keeping some insect populations under control.

(9)The bite of only about 30 species are dangerous, but rarely fatal, to humans.




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