39 { 



THE AGRICULTURAL NEWS. 



December 11, 1909. 



INSECT NOTES. 



NATURAL HISTORY OF INSECTS. 

 Part IV. 1iEsi'ie.\toiiy and Nekvous Sy.stems. 



or are enclo.sed in spots of 



ltESPIE.\TION. 



Insect;; breathe by means of air-tubes (tracheae) which 

 begin at openings in the boily wall, and extend throughout 

 all parts of the in.sect .structure. The openings, which are 

 called spiracles, or stigmata, are valvular, and are capable of 

 being opened and closed. On the outside, they are also 

 generally protected by hairs. There are usually ti-n stigmata 

 on each side of the insect body, but there are sometimes less, 

 and they also vary in their position, according to the life- 

 habits of the insect. They may be distinctly seen with the 

 naked eye in many insects. In certain large caterpillars, 

 they are surrounded by tine lines, 

 colour, which make 

 them prominent In 

 the grasshopper, they 

 may be easily seen: 

 two on the sides of 

 the thorax, and eight 

 on the abdomen The 

 tracheae are composed 

 of chitin, and are con- 

 tinuous with the body 

 wall. The smaller air- 

 tubes are simple 

 tubular structures, 

 but the larger are 

 strengthened on the 

 inside by ridge s, 

 arranged sjrirally. 



The tracheae from 

 the stigmata connect 

 with main line.s, which 

 extend the length of 

 the body, and from 

 which the branches 

 and smaller tubes 

 communicate with all 

 parts. In Fig. 46, the 

 cross-shaded portions 

 are the tracheae, and 

 the very fine cross- 

 lines represent the 

 thickenings mention- 

 ed above. 



In large insects whicli fly 

 flies, butterflies 



Fk;, -IG. TiiAi'iiKAi, System. 



long distances, such a.s pond- 

 and moths, some beetles, flies, bees, etc., 

 there often occur large distensions of certain tracheae to form 

 air sacs, which probably are of use to the insect in making 

 the body more buoyant, and in increasing the rapidity of 

 breathing. 



All insects are air breathers; those which live in the 

 water must come to the surface from time to time for a sup- 

 ply of air, and in many cases the habit of living in water, 

 in decaying organic matter, in plant tissues, or as animal 

 parasites, necessitates some modiKcation, or special adaptation, 

 in the manner of obtaining the necessary air. 



Masquito larvae (Fig. 47) breathe by means of a special 

 organ at the posterior end of the body, which is brought 

 into contact with the air. Maggots of flies which live in 

 plant or animal tissues often have spiracles at the 

 posterior end of the body which communicate with the air. 



Water beetles come to the .surface and carry down, when 

 they dive again, a film of air held by the fine hairs of the 

 body, while certain insects are provided with tracheal gills in 

 the aquatic larval stage. 



Win 



Fir,. 47. Mos()uiTo L.\EVA. 

 The aeration of the blood is accomplished in a manner 

 exactly opposite to that in higher animals. The air penetrates 

 to all parts of the body by means of the tracheae which are 

 bathed by the blood, and comes into intimate contact with 

 the food-substance by means of the minute branches of the 

 tracheae which envelop the alimentary canal. 



XEKVOUS .SY-STEM. 



The relative positions of the vital systems are also 

 opposite in insects to those in higher animals. In insects 

 the circulatory system is dorsal, and the nervous is ventral, 

 while in vertebrate animals the nervous system is dorsal and 

 the circulatory one ventral. 



Id insects, the nervous system is not concenti'ated into 

 a large brain, but is divided among a series of nerve centres, 

 or ganglia, arranged in pairs. The dotted lines in Fig. 45, 

 (see Part III of this paper) .show the position of these 

 ganglia and their connecting nerve cord.s, and Fig. 48 shows 

 the ventral nervous system, with small nerve-branches. 



Fk:. 48. Nervous .System of Insects. 



The double mass shown at the extreme end, and the next 

 portion connected by the heavy lines, together form the brain. 

 The former portion is above the oesophagus or gullet, and 

 the latter belo*, and the heavy lines referred to are nerve 

 cords which connect the two part.s, the gullet runnini' 

 through the opening thus made. The first portion of the 

 nervous system is the supra-oesophageal ganglia, the .second 

 the sub-oeso]>hagcal ganglia; these are connected by the 

 oesophageal nerve collar. Next follow three thoracic and .seven 

 abdominal ganglia, all connected by a central nerve cord. 



The nervous system varies according to general struc- 

 ture, usually by fusion of ganglia, but, with the exception 

 of the supra-oesophageal ganglia, it is always ventral in 

 position. Nerves extend to all the organs and muscles, and' 

 permit the in.sect to respond to a stimulus of any sort. 



The nerve-endings in certain insect structures, such as 

 antennae, palpi, etc., are very delicate, and enable tlie insect 

 to be conscious of very slight stimuli. 



