DISEASES AND INSECTS 



DISEASES AND INSECTS 1037 



1300. A maggot. Larva of 

 a dipterous insect. 



1301. A grub. Larva of 

 a beetle. 



indicate that there were even more kinds of insects 

 then than now. 



Their growth and transformations. Fig. 1296. Insects 



begin life as an egg; in some cases the egg stage is 



passed within the body of the mother, which then 



gives birth to living young. 



j^MtfttM*wgw||to\ The eggs of insects exhibit a 

 >**^ki- vli^o wonderful variety of forms, 



^Mj^^aJ"-^ sizes, colors and characteristic 

 markings. A single scale insect 

 may lay thousands of eggs, while 

 some plant-lice produce only 

 one. Remarkable instinct is 

 often shown by the mother in- 

 sect in placing her eggs where 

 her young will find proper 

 food. From their birth the 

 ! young of some of the lowest or 

 most generalized insects closely 

 resemble their parents, and 

 they undergo no striking change 

 during their life; hence are said 

 to have no metamorphosis. In 

 the case of grasshoppers, stink- 

 bugs, dragon-flies, and many 

 othe insects, the young at birth resemble their par- 

 ents, but have no wings. As they grow, wings gradu- 

 ally develop and often changes in markings occur, until 

 the adult stage is. reached. The growth, however, is 

 gradual, and no striking or complete change occurs, and 

 these insects are said to undergo an incomplete meta- 

 morphosis. The young insects in all stages are called 

 nymphs (Fig. 1297) ; thus insects with an incom- 

 plete metamorphosis pass through three different 

 forms during their life: an egg, the young or 

 nymph stage, and the adult. From the eggs of 

 butterflies, moths, flies, beetles, bees and some 

 other insects, there hatches a worm-like crea- 

 ture, much unlike the parent insect. It is called 

 a larva (Fig. 1298) ; the larvae of 

 butterflies and moths are often 

 called caterpillars (Fig. 1299) ; mag- 

 gots are the larvae of flies (Fig. 

 1300) ; and the term grub is applied 

 to the larvae of beetles 

 and bees (Fig. 1301). 

 When these larvae get 

 their full growth, some 

 of them go into the 

 ground where they 

 form an earthen cell, 

 while others proceed 

 to spin around them- 

 selves a silken home 

 or cocoon (Figs. 1302- 

 1304). In these re- 

 treats the larvae change 

 to a quiescent or life- 

 less-appearing creature 

 which has little resem- 

 blance to either the 

 larva or the parent 

 insect. It is call a 

 pupa (Fig. 1305). The 

 pupae of butterflies are 

 often called chry solids. Flies change to pupae in the 

 hardened skin of the maggot. Some pupae, like those 

 of mosquitos, are very active. Wonderful changes 

 take place within the skin of the pupa. Nearly all the 

 larval tissues break down and the insect is practically 

 made over, from a crawling larva to a beautiful, 

 flying adult insect. When the adult is fully formed, 

 it breaks its pupal shroud and emerges to spend a 

 comparatively brief existence as a winged creature. 

 Such insects are said to undergo a complete metamor- 

 phosis, and pass through four strikingly different 



1302. Cocoon of Pro- 



methea moth. 

 Made in the roll of a 

 leaf. The insect weaves a 

 web about the leaf-stalk 

 and ties it to the parent 

 stem, so that the leaf 

 cannot falL 



stages during their life: the egg, the worm- 

 like larva, the quiescent pupa, and the 

 adult insect. Such remarkable changes 

 or transformations make the story of an 

 insect's life one of intense interest to one 

 who reads it from nature's book. Vari- 

 ous kinds of adult insects, or imagoes, are 

 shown in Figs. 1306-1311. No two kinds 

 of insects have the same life-story to tell. 

 Some pass their whole life on a single 

 host; some partake of only a certain kind 

 of food, while others thrive on many kinds 

 of plants; some are cannibals at times, 

 and others, like the parasites, are boarders 

 within their host, while many prey openly 

 on their brethren in the insect world. 

 Usually the life of the adult insect is brief, 

 but ants have been kept for thirteen 

 years, and the periodical cicada has to 

 spend seventeen years as a nymph under- 

 ground before it is fitted to become a 

 denizen of the air. The winter months 

 op e n i n g m ay be passed in any of the different 



t n r o u g n a + anoa ^.f +V.Q ino/>4-'d Mtn TWr. ,,..,, 

 which the 



C tn . e msec t's life. Two very 

 moth escaped, closely allied insects may have very differ- 

 ent life habits. 



How they grow. Many persons think that the small 

 house-flies grow to be the large ones. While most 

 insects feed after they become adults, they get little or 

 none of their growth during their adult life. Insects 

 grow mostly while they are 

 larvae, or nymphs. The maggots 

 from which the little house-flies 

 develop doubtless do not have 

 as luxuriant or favorable feed- 

 ing-grounds as do those of the 

 larger flies. In thirty days some 

 leaf-feeding caterpillars will in- 

 crease in size 10,000 times; and 

 a certain flesh-feeding maggot 

 will in twenty -four hours con- 

 sume two hundred times its own 

 weight, which would be paral- 

 leled in the human race if a one- 

 day-old baby ate 1,500 pounds 

 the first day of its existence! 

 The skin of insects is so hard 

 and inelastic that it cannot 

 stretch to accommodate such rapid growth. But 

 nature obviates this difficulty by teaching these crea- 

 tures how to grow a new suit of clothes or a new skin 

 underneath the old one, and then to shed or molt the 

 latter. The old skin is shed in its entirety, even from 

 all the appendages, and sometimes remains in such a 

 natural position where the insect left it as to easily 

 deceive one into thinking that he is looking at the 

 insect rather than at its cast-off clothes. Some insects 

 are so neat and economical that they devour their old 

 suits or skins soon after molting them. 

 Larvae, or nymphs, may molt from two 

 or three to ten or more times; the larvae 

 do not often change strikingly in appear- 



1304. End of cocoon of 



Cecropia moth. 

 Inside view, showing 

 where the moth gets out. 



1305. Pupa of 

 tomato worm. 



1306. The cabbage butterfly. 



