106 ^ 



THE PRACTICAL ENTOMOLOGIST. 



Ontlines of the Study of Insects. — III. 



Having sLewn how the Class of Insects may be 

 known from the Classes of Crustacea and Worms, it 

 remains for us to treat of the sub-divisions of the 

 Insects. Of these there are three grand groups 

 or Orders ; namely, the Centipedes and " Thous- 

 and-legs" or Jli/n'ajwda ; the Spiders, Scorpions and 

 Mites, or AracJtniJo. and the true sis-legged In- 

 sects. The Myriapoda are the lowest order by rea- 

 son of their wonn-like form. Each ring or seg- 

 ment repeats the form of that next to it, is provi- 

 ded with a pair of legs, from the head to the op- 

 posite end of the body, and the distinctions between 

 the head, thorax and abdomen are so slight .is to 

 be easily passed over. But in the young, the body 

 is more insect-like, there is a distinct head, and the 

 three pairs of legs attached to the thoracic region, 

 together with the absence of abdominal legs, all 

 show that the adult Centipede is essentially an insect, 

 and does not belong to an independent class, as 

 taught in most of the text books. 



The Myriapods, with some Spiders, also diflFer 

 from all others of the Class, in that the j'olk is not 

 enclosed within the body walls until sometime after 

 the larva is hatched. The 3Iyriapods are, in some 

 genera — such as the Centipede — provided with poi- 

 son sacs at the base of their powerful jaws, which 

 render them a terror to man. They are not pro- 

 vided with compound eyes, and only have simple 

 eyelets, thus agreeing with the Spiders in this re- 

 spect. The young larva attains its growth bj- the 

 addition of rings, often more than 100 in number, 

 which grow out from just in front of the second 

 ring from the end of the abdomen. 



The ^Myriapods are divided into two .suborders ; 

 i. e. those genera, such as the Centipedes, which 

 are provided with flat bodies and have few rings, 

 and consequently a less number of legs than the 

 '•Thousand-legs," or Jitliis, comprising the higher 

 suborder; while the latter, whose bodies are cj-lin- 

 drical and many jointed, with verj- numerous legs, 

 form the second and lower suborder. 



The second and next higher order of Insccfo, are 

 the Arachnula or Spiders* Scorpions and Mites. 

 AVhile the three regions of the body are here pre- 

 sent, the head is so merged iu the thorax as to re- 

 semble strikingly the Crab, with its head-thorax 

 and abdomen, and in the larger number of species, 

 the abdomen is of great size when compared with the 

 I'.ead and thorax, as seen iu the common IIou.<o 

 Spider. But when we study the development of 

 the Spider, we find that at first the embryo is worm- 

 like, and that many species of mites are at first six- 

 legged, though the adults arc invariably provided 

 with eight legs, and that the head is at first very 

 distinct from the thorax. Thus the Spiders and 

 Myriapods first start as worms, then become like the 

 true insects in form, but when full grown, by a re- 

 trograde process of growth, lose their essential in- 

 sect form and assume .shapes which remind us of 

 the much lower Worms and Crustacea. 



The Spiders have no antenna;, and genorally 4 

 pairs of simple eyes ; their bite is poisonous to in- 



sects and other animals on which they prey, but 

 rarely to man. 



After leaving the egg, Spiders grow by succes- 

 sive moultings, or changes of skin, not passing 

 through, as a rule, successive metamorphoses like 

 the insects, though the lowest Arachnida, such as 

 the parasitic mites, do undergo a distinct metamor- 

 phosis. 



The Arachnida possess four pairs of thoracic 

 feet; and the three pairs of spinnerets at the extre- 

 mity of the abdomen are but modified limbs. The 

 silk glands, situated in the abdomen, are also six in 

 number, and in Kpeira, the common Garden Spi- 

 der, which has a very large abdomen and spins 

 large webs, they occupy one-fourth of the abdomi- 

 nal cavit_y. Each gland terminates by means of a 

 slender tube in one of the six spinnerets. The 

 males are generally very difl'erent from the females, 

 differing in this respect from the hexapodous, or 

 six-legged Insects. 



Spiders breathe by means of trachese and also 

 by gills, or so-called Pulmonary Sacs. The Pul- 

 monari'a, or true Spiders and Scorpions, are provi- 

 ded with from six to twelve ocelli, and breathe by 

 rerial gills; while the Tnuhean'a, or Mites, inclu- 

 ding the False Scorpions, such as Chelifer, a mi- 

 nute scorpion-like mite found in dusty places; and 

 the Harvest Men, often called Daddy-long-legs, 

 found iu damp shady places, breathe by tracheje. 

 The mites are usually parasitic on other animals, 

 including man. Ixodes, the wood-tick, lives in 

 forests, and attaches itself to passing animals. It is 

 found in warm climates. Some low Mites which 

 live as parasites on other, mostly aquatic, animals, 

 are so low and degraded, from Being stationary, in 

 their habits, and little short of mere machines for 

 sucking the juices of their victims; as to be scarce- 

 ly distinguishable from low Crustacea of similar 

 habits. Thus two great divisions of articulates, 

 really so different in the essential plan of their 

 structure, by a similar mode of life assume nearly 

 identical forms, just as the warm blooded whale 

 which sucks its young, assumes the form of the 

 cold blooded fish. 



We have seen that in the true six-legged, flying 

 insects, each of the three regions of the body has 

 become more specialized than in the Myriapods or 

 Spiders. The head is freer from the body, and 

 nearly eqtials in size the thorax. 



The head of insects, though generally supposed 

 to be composed of but a single ring, equivalent to 

 one of the thoracic or abdominal segments, is in 

 reality probably composed of seven such rings, of 

 which four are grouped in front of the mouth-open- 

 ing, and three behind. The four rings in front, 

 bear the organs of sight and sen.sation, such as the 

 ocelli or simple eyes, of which there are probably 

 two pairs, and the compound eyes, and the an- 

 tennne, while the three rings behind the mouth bear 

 the palpi, and two pairs of jaws, which are for di- 

 gestive purposes, i. e. seizing and preparing the , 

 food for digestion in the stomach. 



There are both simple and compound eyes, the 

 latter composed of thousands of facets which are 



