**REFERENCES**

[1] D. Bai and Y. Xu, Periodic solutions of first order functional differential equations with periodic

deviations, Comp. Math. Appl. 53(2007), 1361–1366.

[2] S. Cheng, G. Zhang, Existence of positive periodic solutions for non-autonomous functional

differential equations, Electron. J. Differrential Equations 59 (2001), 1–8.

[3] K. Gopalsamy, Stability and oscillation in delay differential equations of population dynamics,

Kluwer Academic Press, Boston, 1992.

[4] K. Gopalsamy, S. I. Trofimchuk, Almost periodic solutions of Lasota-Wazewska type delay

differential equations, J. Math. Anal. Appl. 237 (1999), 106–123.

[5] K. Gopalsamy and P. Weng, Global attractivity and level crossing in model of Hematopoiesis,

Bull. Inst. Math. Acad. Sinica 22 (1994), 341–360.

[6] J. R. Graef, C. Qian and P. W. Spikes, Oscillation and global attractivity in a periodic delay

equation, Canad. Math. Bull. 38 (3) (1996), 275–283.

[7] W. S. C. Gurney, S. P. Blathe and R. M. Nishet, Nicholson’s blowflies revisited, Nature 287

(1980), 17–21.

[8] D. Jiang, J. Wei, B. Jhang, Positive periodic solutions of functional differential equations and

populations models, Electron. J. Differential Equations 71(2002), 1-13.

[9] W. Joseph, H. So, J. Wu and X. Zhous, Structured population on two patches: modelling

desparel and delay, J. Math. Biology 43 (2001), 37–51.

[10] W. Joseph, H. So and J. Yu, Global attractivity and uniform stability in Nicholson’s blowflies,

Differential Equations Dynam. Systems 1 (1994), 11–18.

[11] Y. Kuang, Delay differential equations with applications in population dynamics, Academic

Press, New York, 1993.

[12] R. W. Leggett, L. R. Williams, Multiple positive fixed points of nonlinear operators on ordered

Banach spaces, Indiana Univ.Math.J. 28 (1979), 673–688.

[13] B. S. Lalli and B. G. Ghang, On a periodic delay population model, Quart. Appl. Math. LII

(1994), 35–42.

[14] Y. Li, Existence and global attractivity of positive periodic solutions of a class of delay differential

equations, Science in China (Series) 41 (3) (1998), 273–284.

[15] J. Lio and J. Yu, Global asymptotic stability of nonautonomous mathematical ecological equations

with distributed deviating arguments (in chinese), Acta.Math. Sinica 41 (1998), 1273–

1282.

[16] Y. Luo, W. Wang and J. Shen, Existence of positive periodic solutions for two kinds of neutral

functional differential equations, To appear in Appl. Math. Lett.

[17] A. J. Nicholsons, The balance of animal population, J.Animal Ecology 2 (1993), 132–178.

[18] Seshadev Padhi and Shilpee Srivastava, Multiple periodic solutions for nonlinear first order

functional differential equations with applications to population dynamics, Appl. Math. Comput.

(2008), doi:10.1016/j.amc.2008.03.031.

[19] F. Xiang and Y. Rang, On the Lasota-Wazewska model with piecewise constant arguments,

Acta. Math. Scientia 26B (2) (2006), 371–378.

[20] H. Wang, Positive periodic solutions for functioanl differential equations, J.Differential Equations

202 (2004), 354–366.

[21] A. Wan, D. Jiang, Existence of positive periodic solutions for functional differential equations,

Kyushu J.Math. 56 (2002), 193–202.

[22] A. Wan, D. Jiang, A new existence theory for positive periodic solutions to functional differential

equations, Comp. Math. Appl. 47 (2004), 1257–1262.

[23] M. Wazewska-Czyzewska and A. Lasota, Mathematical problems of the dyanamics of red blood

cells systems, Ann. Polish Math. Soc. Series III, Appl. Math. 17 (1988), 23–40.

[24] P. Weng, M. Liang, The existence and behaviour of periodic solutions of Hematopoiesis model,

Math. Appl. 4 (1995), 434–439.

[25] G. Zhang, S. Cheng, Positive periodic solutions of nonautonomuos functioanal differential

equations depending on a parameter, Abstract Appl. Anal. 7 (2002), 279–286.

[26] W. Zhang, D. Zhu, P. Bi, Existence of periodic solutions of a scalar functional differential

equations via a fixed point theorem, Math. Comput. Modelling 46 (2007), 718–729.