94 



THE PRACTICAL ENTOMOLOGIST. 



Outlines of the Study of Insects. — II. 



In our last article we endeavored to show that the 

 classes of Worms, Crustacea and Insects were but 

 modifications of a single articulated or jointed 

 worm-like form, and that in all this unity there ac- 

 tually existed from the first, three quite different 

 ti/pea or shapes, so that one need not confuse a 

 worm with a Lobster, or a Lobster with a Honey bee. 

 At first however, all look much alike, that is when 

 in the egg, before the embryo is formed. The ear- 

 liest form of all articulates is wormlike, that is, the 

 embryos tend to become cylindrical, much longer 

 than broad, and rounded off much alike at each end 

 of the body. 



In studying the lives of great men, we turn 

 eagerly to the accounts of their childhood, and 

 growth through the period of youth to adult life. 

 So in studying insects we must trace them from 

 the egg state, to the period of childhood, (the lar- 

 va) — and from childhood to the adult fly-state. 



At first the Worm leaves the egg as a little oval 

 infusorium-like, microscopic body, covered over with 

 little filaments or cilia, by the swift motion of which 

 it circumnavigates a drop of water. Very soon it 

 grows longer, and contracts at intervals, when the 

 skin becomes partially infolded, giving it a ringed 

 appearance, and it is at this time that we can tell 

 Tvhether the embryo is to be a lobster or worm, i. e. 

 an articulate, or a clam or fish. The rings faintly 

 marked olit determine that in reality it is an Arti- 

 culate and not a Mollusc (clam), or Radiate (Star- 

 fish). Soon the ciliae disappears, regular locomo- 

 tive paddle-like organs, grow out from the sides; 

 feelers, and jaws and eye-dots appear on the front 

 rings of the body, which are thus grouped into a 

 head, though it is diificult in a large proportion of 

 the lower kinds of worms for unskilled observers to 

 distinguish the head from the tail. Thus we see 

 throughout the growth of a worm, no attempt at 

 subdividing the body into regions, as a head, tho- 

 rax and abdomen, each provided with distinct or-- 

 gans, but only a perfection of the individual rings 

 themselves as they advance in life. Thus in a 

 worm, of which fig. 1, in the preceding article is a 

 cross section, we see each ring is plainly distin- 

 guished into an upper and under, and in addition 

 to these, a well marked side-area, from which oar- 

 like locomotive organs grow out. It is on this side 

 area in the rings composing the head of worms, 

 that the eye-dots, feelers and jaws are situated. We 

 see this arrangement distinctly in the Worms, but 

 less apparently in the Crustacea and Insects, whose 

 heads are made up not of a single ring, as is very 

 generally supposed, but of six and probahly seven, 

 being in insects twice the number of those com- 

 prising the thorax. 



In some of the low intestinal worms, such as the 

 Tape Worm, each ring in the hinder two-thirds of 

 the body is provided with a distinct portion of the 

 ovary and male sperm-gland, so that when the body 

 becomes broken up into its constituent elements or 



rings, as often occurs naturally in these low forms 

 for the more ready propagation of the species, since 

 the young are exposed to many dangers while liv- 

 ing ill the intestines of animals, — each ring may 

 become a living independent worm, and add new 

 rings to its body by the simple subdivision of the 

 original one. This fact proves that in the worm, 

 the vitality of the animal is very equally distributed 

 to each ring. If we cut off the head or tail of some 

 of the lowest of these worms, the pieces will become 

 a distinct animal, but an insect or crab sooner or later 

 dies when deprived of its head or tail, (abdomen). 

 The young of all Crustacea first begin life in the 

 egg as oblong flattened worm-like bodies, each end 

 of the body repeating the same form. The young 

 of the low Crustacea, such as the Barnacles, and 

 some marine forms like the Sow-bugs, and some lowly 

 organized parasitic species inhabiting the gills of 

 fishes, are hatched as microscopic embryos which 

 would readily be mistaken for young worms. In 

 the higher Crustacea such as the fresh-water lob- 

 ster, the young when hatched, does not greatly dif- 

 fer from the parent, but it goes through the worm- 

 like stage within the egg. 



It is thus with Insects. Within the egg at the 

 first dawn of life they are flattened oblong bodies 

 curved upon themselves. Just before hatching 

 their bodies unbend, they become more cylindrical, 

 the limbs bud out on the sides of the rings, the 

 head is clearly demarked, and the young Caterpil- 

 lar steps forth from the egg-shell ready armed and 

 equipped for its riotous life. 



As will be seen by the figures below, the legs, 

 jaws and antennse are first started as buds from the 

 side of the rings, being simply elongations of the 

 body wall, which bud out, become larger, and fi- 

 nally jointed, until the buds arising from the 

 thorax or abdomen become legs, those from the 

 base of the head become jaws, while the antennas 

 and palpi sprout out from the front rings of the 

 head. Thus while the body of all articulates are 

 built up from a common embryonic form, their ap- 

 pendages also so diverse, when we compare a lob- 

 ster's claw with an insect's antennas, or a spider's 

 spinneret with the hinder limbs of a Centipede, are 

 yet but modifications of a common form, adapted 

 for the different uses to which they are put by these 

 animals. 



The changes which an animal undergoes while 

 growing is called its metamorphosis. This meta- 

 morphosis is of course less marked in those animals 

 whose forms are low and simple and which there- 

 fore do not depart greatly from the early embry- 

 onic stage. As we ascend higher, the changes be- 

 come more marked, until in the true Insects, where 

 the larva is so strikingly different in the majority 

 of species, from the pupa, while the pupa again, as- 

 sumes quite a different form from the winged fly 

 we have a most thorough transformation, which led 

 early observers to state that Insects differ from 

 other animals in passing through a metamorphosis, 

 not knowing as we do now, what surprising changes 

 all animals pass through in order to reach the 

 adult state. 



